| Issue |
204270
|
| Summary |
[ARM] __builtin_subc() does not generate SBC instruction
|
| Labels |
|
| Assignees |
|
| Reporter |
calc84maniac
|
On 32-bit ARM, `__builtin_addc()` works to generate ADC, but `__builtin_subc()` appears to always perform two integer subtractions. I found an open PR that seems to address a similar optimization issue, though I'm not sure if it directly fixes this: https://github.com/llvm/llvm-project/pull/184021
https://godbolt.org/z/MYdMY85TK
```c
unsigned long long add64(unsigned x0, unsigned x1, unsigned y0, unsigned y1)
{
unsigned carry = 0;
x0 = __builtin_addc(x0, y0, carry, &carry);
x1 = __builtin_addc(x1, y1, carry, &carry);
return (unsigned long long)x1 << 32 | x0;
}
unsigned long long sub64(unsigned x0, unsigned x1, unsigned y0, unsigned y1)
{
unsigned carry = 0;
x0 = __builtin_subc(x0, y0, carry, &carry);
x1 = __builtin_subc(x1, y1, carry, &carry);
return (unsigned long long)x1 << 32 | x0;
}
```
```gas
add64:
adds r0, r0, r2
adc r1, r3, r1
bx lr
sub64:
sub r1, r1, r3
mov r3, #0
subs r0, r0, r2
adc r2, r3, #0
eor r2, r2, #1
sub r1, r1, r2
bx lr
```
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