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https://github.com/pkivolowitz/asm_book.git
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added complex ARM instructions
This commit is contained in:
Perry Kivolowitz
parent
b3f1a63e51
commit
743550c02d
6 changed files with 234 additions and 140 deletions
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@ -212,12 +212,72 @@ side effect to changing this order.
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`lsl` means "left shift logical" which fills the right side recently
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vacated bits with zero.
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Now, we present a more sophisticated version of `SetB`:
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```asm
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SetB: ldrb w3, [x0] // 1
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bfi w3, w1, 1, 2 // copy bit 0..1 in w1 to bit 1..2 in w3 // 2
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strb w3, [x0] // 3
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ret // 4
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```
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Whoa. Nine instructions down to four! What the heck is `bfi`?
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`bfi dst, src, start, width` copies `width` bits starting at 0 in `src`
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to bits starting at `start` in `dst`. It obviates the need for `line 2` in
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the naive code because it plucks only bits 0 and 1 from the original value
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of `w1`. The `bfi` then internally does the shift appropriate to move
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bit 0 of the original `w1` to bit `start` along with `width - 1`
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subsequent bits.
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Some might argue that instructions like `bfi` (and `ubfiz` described
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below) is an example of `ISA creep` where ISA's get
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more and more cumbersome with the latest instructions du jure. This is
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definitely true in the x86 ISA. Perhaps this is true in the AARCH64 ISA
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as well, but certainly not to the extent of the x86. Remember that the ARM
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family of processors are examples of RISC machines - *reduced instruction
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set* architectures.
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UBFIZ dest, src, start, width
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zeros dest
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copies src starting at 0 to bits start to start + width - 1.
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Notice this version is two instructions shorter.
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Part of the savings is the use of `ubfiz`.
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`ubfiz` stands for Unsigned Bit Field Insert in Zeros. Wow.
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This instruction does the following:
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* zeros the entire destination register
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* copies the indicated source register bits to the destination
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Finally, we come to handling field `c`. Recall `c` is 5 bits long starting
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at bit 3.
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Clearing the bits in `c` is easily accomplished:
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```c
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void ClearC(unsigned char * byte) {
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*byte &= 7; // squashes bits 3 to 7 to 0
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}
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```
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This is optimally implemented using:
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```asm
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ClearC: ldrb w1, [x0] // 1
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and w1, w1, 7 // 2
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strb w1, [x0] // 3
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ret // 4
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```
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```c
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void SetC(unsigned char * byte, unsigned char value) {
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value &= 0x1F; // ensures only bits 0 to 4 can be set
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*byte &= ~(0x1F << 3); // squashes correct bits in byte
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@ -228,7 +288,23 @@ void SetC(unsigned char * byte, unsigned char value) {
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In naive assembly language, these functions would look like this:
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```asm
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SetC: ldrb w3, [x0] // 1
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mov w2, 0x1F // 2
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and w1, w1, w2 // 3
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lsl w1, w1, 3 // 4
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lsl w2, w2, 3 // 5
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mvn w2, w2 // 6
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and w3, w3, w2 // 7
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orr w3, w3, w1 // 8
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strb w3, [x0] // 9
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ret // 10
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```
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```asm
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SetC: ldrb w2, [x0] // put *byte into w2 // 1
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ubfiz w1, w1, 3, 5 // zero new w1, copy bits 0..4 to 3..7 // 2
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and w2, w2, 7 // preserve only 1st 3 bits in *byte // 3
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orr w2, w2, w1 // or in value into *byte // 4
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strb w2, [x0] // 5
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ret // 6
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```
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@ -14,7 +14,8 @@ ClearA: ldrb w1, [x0]
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ret
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ClearB: ldrb w1, [x0]
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and w1, w1, 0xF9
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mov w2, 0xF9
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and w1, w1, w2
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strb w1, [x0]
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ret
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@ -30,27 +31,15 @@ SetA: ldrb w1, [x0]
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SetB: ldrb w3, [x0]
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and w1, w1, 3 // value &= 3
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lsl w1, w1, 1
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mov w2, 6
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mvn w2, w2
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and w3, w3, w2 // B is cleared
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orr w3, w3, w1
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bfi w3, w1, 1, 2 // copy bit 0..1 in w1 to bit 1..2 in w3
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strb w3, [x0]
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ret
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SetC: ldrb w3, [x0]
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mov w2, 0x1F
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and w1, w1, w2
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lsl w1, w1, 3
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lsl w2, w2, 3
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mvn w2, w2
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and w3, w3, w2
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orr w3, w3, w1
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strb w3, [x0]
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SetC: ldrb w2, [x0] // put *byte into w2
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ubfiz w1, w1, 3, 5 // zero new w1, copy bits 0..4 to 3..7
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and w2, w2, 7 // preserve only 1st 3 bits in *byte
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orr w2, w2, w1 // or in value into *byte
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strb w2, [x0]
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ret
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.end
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@ -1,10 +1,5 @@
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SetB: ldrb w3, [x0]
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and w1, w1, 3 // value &= 3
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lsl w1, w1, 1
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mov w2, 6
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mvn w2, w2
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and w3, w3, w2 // B is cleared
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orr w3, w3, w1
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bfi w3, w1, 1, 2 // copy bit 0..1 in w1 to bit 1..2 in w3
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strb w3, [x0]
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ret
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@ -8,7 +8,8 @@ struct BF {
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unsigned char noBF = 0;
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#define C
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//#define C
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#undef C
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#ifdef C
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/* Note the absence of defensive programming such as checking
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to ensure that byte is not null and that bit_number is not
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@ -59,11 +60,11 @@ int main() {
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bf.b = 2;
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bf.c = 3;
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ClearA(&noBF);
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SetB(&noBF, 1);
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SetC(&noBF, 7);
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SetA(&noBF);
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SetB(&noBF, 2);
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SetC(&noBF, 3);
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printf("noBF should be 0x3A - value: 0x%X\n", (unsigned int) noBF);
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printf("noBF should be 0x1D - value: 0x%X\n", (unsigned int) noBF);
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printf("bf should be 0x1D - value: 0x%X\n", (unsigned int) *((unsigned char *) &bf));
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return 0;
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}
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@ -1,129 +1,151 @@
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.section __TEXT,__text,regular,pure_instructions
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.build_version macos, 12, 0 sdk_version 12, 3
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.globl _ClearA ; -- Begin function ClearA
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.p2align 2
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_ClearA: ; @ClearA
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.arch armv8-a
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.file "test.c"
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.text
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.align 2
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.p2align 3,,7
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.global ClearA
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.type ClearA, %function
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ClearA:
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.LFB23:
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.cfi_startproc
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; %bb.0:
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ldrb w8, [x0]
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and w8, w8, #0xfe
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strb w8, [x0]
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ldrb w1, [x0]
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and w1, w1, -2
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strb w1, [x0]
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ret
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.cfi_endproc
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; -- End function
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.globl _SetA ; -- Begin function SetA
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.p2align 2
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_SetA: ; @SetA
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.LFE23:
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.size ClearA, .-ClearA
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.align 2
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.p2align 3,,7
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.global SetA
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.type SetA, %function
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SetA:
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.LFB24:
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.cfi_startproc
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; %bb.0:
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ldrb w8, [x0]
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orr w8, w8, #0x1
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strb w8, [x0]
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ldrb w1, [x0]
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orr w1, w1, 1
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strb w1, [x0]
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ret
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.cfi_endproc
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; -- End function
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.globl _ClearB ; -- Begin function ClearB
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.p2align 2
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_ClearB: ; @ClearB
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.LFE24:
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.size SetA, .-SetA
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.align 2
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.p2align 3,,7
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.global ClearB
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.type ClearB, %function
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ClearB:
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.LFB25:
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.cfi_startproc
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; %bb.0:
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ldrb w8, [x0]
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and w8, w8, #0xfffffff9
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strb w8, [x0]
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ldrb w1, [x0]
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and w1, w1, -7
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strb w1, [x0]
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ret
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.cfi_endproc
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; -- End function
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.globl _SetB ; -- Begin function SetB
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.p2align 2
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_SetB: ; @SetB
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.LFE25:
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.size ClearB, .-ClearB
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.align 2
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.p2align 3,,7
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.global SetB
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.type SetB, %function
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SetB:
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.LFB26:
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.cfi_startproc
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; %bb.0:
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ldrb w8, [x0]
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and w8, w8, #0xfffffff9
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ubfiz w9, w1, #1, #2
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orr w8, w8, w9
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strb w8, [x0]
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ldrb w2, [x0]
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bfi w2, w1, 1, 2
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strb w2, [x0]
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ret
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.cfi_endproc
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; -- End function
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.globl _ClearC ; -- Begin function ClearC
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.p2align 2
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_ClearC: ; @ClearC
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.LFE26:
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.size SetB, .-SetB
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.align 2
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.p2align 3,,7
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.global ClearC
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.type ClearC, %function
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ClearC:
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.LFB27:
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.cfi_startproc
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; %bb.0:
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ldrb w8, [x0]
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and w8, w8, #0x7
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strb w8, [x0]
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ldrb w1, [x0]
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and w1, w1, 7
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strb w1, [x0]
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ret
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.cfi_endproc
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; -- End function
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.globl _SetC ; -- Begin function SetC
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.p2align 2
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_SetC: ; @SetC
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.LFE27:
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.size ClearC, .-ClearC
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.align 2
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.p2align 3,,7
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.global SetC
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.type SetC, %function
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SetC:
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.LFB28:
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.cfi_startproc
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; %bb.0:
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ldrb w8, [x0]
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bfi w8, w1, #3, #8
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strb w8, [x0]
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ldrb w2, [x0] // load *byte into w2
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ubfiz w1, w1, 3, 5 // put zeros in bit 3 to 7 of value
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and w2, w2, 7 // blank bits 3 to 7 in *byte
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orr w2, w2, w1
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strb w2, [x0]
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ret
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.cfi_endproc
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; -- End function
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.globl _main ; -- Begin function main
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.p2align 2
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_main: ; @main
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.LFE28:
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.size SetC, .-SetC
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.section .rodata.str1.8,"aMS",@progbits,1
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.align 3
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.LC0:
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.string "noBF should be 0x3A - value: 0x%X\n"
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.align 3
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.LC1:
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.string "bf should be 0x1D - value: 0x%X\n"
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.section .text.startup,"ax",@progbits
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.align 2
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.p2align 3,,7
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.global main
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.type main, %function
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main:
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.LFB29:
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.cfi_startproc
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; %bb.0:
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sub sp, sp, #48
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stp x20, x19, [sp, #16] ; 16-byte Folded Spill
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stp x29, x30, [sp, #32] ; 16-byte Folded Spill
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add x29, sp, #32
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.cfi_def_cfa w29, 16
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.cfi_offset w30, -8
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.cfi_offset w29, -16
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.cfi_offset w19, -24
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.cfi_offset w20, -32
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ldrb w8, [sp, #15]
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orr w8, w8, #0x1
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strb w8, [sp, #15]
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ldrb w8, [sp, #15]
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and w8, w8, #0xfffffff9
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orr w8, w8, #0x4
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strb w8, [sp, #15]
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ldrb w8, [sp, #15]
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mov w19, #24
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bfxil w19, w8, #0, #3
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strb w19, [sp, #15]
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mov w8, #58
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adrp x9, _noBF@PAGE
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strb w8, [x9, _noBF@PAGEOFF]
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str x8, [sp]
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Lloh0:
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adrp x0, l_.str@PAGE
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Lloh1:
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add x0, x0, l_.str@PAGEOFF
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bl _printf
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str x19, [sp]
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Lloh2:
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adrp x0, l_.str.1@PAGE
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Lloh3:
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add x0, x0, l_.str.1@PAGEOFF
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bl _printf
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mov w0, #0
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ldp x29, x30, [sp, #32] ; 16-byte Folded Reload
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ldp x20, x19, [sp, #16] ; 16-byte Folded Reload
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add sp, sp, #48
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stp x29, x30, [sp, -32]!
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.cfi_def_cfa_offset 32
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.cfi_offset 29, -32
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.cfi_offset 30, -24
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mov w5, 2
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mov w4, 3
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mov x29, sp
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ldrb w0, [sp, 24]
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adrp x2, .LANCHOR0
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adrp x1, .LC0
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add x1, x1, :lo12:.LC0
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orr w0, w0, 1
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strb w0, [sp, 24]
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mov w0, 58
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strb w0, [x2, #:lo12:.LANCHOR0]
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ldrb w3, [sp, 24]
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mov w2, 58
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mov w0, 1
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bfi w3, w5, 1, 2
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strb w3, [sp, 24]
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ldrb w3, [sp, 24]
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bfi w3, w4, 3, 5
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strb w3, [sp, 24]
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bl __printf_chk
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ldrb w2, [sp, 24]
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adrp x1, .LC1
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mov w0, 1
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add x1, x1, :lo12:.LC1
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bl __printf_chk
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mov w0, 0
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ldp x29, x30, [sp], 32
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.cfi_restore 30
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.cfi_restore 29
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.cfi_def_cfa_offset 0
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ret
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.loh AdrpAdd Lloh2, Lloh3
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.loh AdrpAdd Lloh0, Lloh1
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.cfi_endproc
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; -- End function
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.globl _noBF ; @noBF
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.zerofill __DATA,__common,_noBF,1,0
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.section __TEXT,__cstring,cstring_literals
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l_.str: ; @.str
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.asciz "noBF should be 0x3A - value: 0x%X\n"
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l_.str.1: ; @.str.1
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.asciz "bf should be 0x1D - value: 0x%X\n"
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.subsections_via_symbols
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.LFE29:
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.size main, .-main
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.global noBF
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.bss
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.set .LANCHOR0,. + 0
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.type noBF, %object
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.size noBF, 1
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noBF:
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.zero 1
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.ident "GCC: (Ubuntu 9.4.0-1ubuntu1~20.04.1) 9.4.0"
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.section .note.GNU-stack,"",@progbits
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11
section_2/bitfields/ubfiz.s
Normal file
11
section_2/bitfields/ubfiz.s
Normal file
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@ -0,0 +1,11 @@
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.global main
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.text
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.align 2
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main: str x30, [sp, -16]!
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mov w1, 0xFF
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mov w2, 0x0A
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ubfiz w1, w2, 0, 4
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ldr x30, [sp], 16
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mov w0, wzr
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ret
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