[AArch64] Eliminate cmp by reassocating add and sub
#119606
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@llvm/issue-subscribers-backend-aarch64 Author: Karl Meakin (Kmeakin)
This rust fn:
https://godbolt.org/z/5EKvWeEYb #[no_mangle]
pub fn last(xs:&[u8])->Option<&u8>{
xs.last()
}produces this assembly: last:
add x8, x1, x0
cmp x1, #<!-- -->0
sub x8, x8, #<!-- -->1
csel x0, xzr, x8, eq
retEquivalent C also produces the same assembly #include <stddef.h>
#include <stdint.h>
uint8_t* last(uint8_t* x0, size_t x1) {
if (x1 == 0) {
return NULL;
} else {
return x0 + x1 - 1;
}
}By reassocating tgt: // @<!-- -->tgt
subs x8, x1, #<!-- -->1
add x8, x0, x8
csel x0, xzr, x8, lo
retI could not convince clang to produce this assembly, even using #include <stdbool.h>
#include <stddef.h>
#include <stdint.h>
uint8_t* tgt(uint8_t* x0, size_t x1) {
size_t sum;
bool overflow = __builtin_sub_overflow(x1, 1, &sum);
if (overflow) {
return NULL;
} else {
return x0 + sum;
}
}I could only produce this assembly by writing the LLVM IR manually: define noundef i64 @<!-- -->tgt(i64 noundef %0, i64 noundef %1) {
%4 = tail call { i64, i1 } @<!-- -->llvm.usub.with.overflow.i64(i64 %1, i64 1)
%overflow = extractvalue { i64, i1 } %4, 1
%diff = extractvalue {i64, i1} %4, 0
%sum = add i64 %0, %diff
%ret = select i1 %overflow, i64 0, i64 %sum
ret i64 %ret
} |
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This rust fn:
https://godbolt.org/z/W9WM6YPYf
produces this assembly:
Equivalent C also produces the same assembly
https://godbolt.org/z/c5dMbv63K
By reassocating
subandadd(from(x0 + x1) -1tox0 + (x1 - 1)), we can save an instruction by reusing the overflow flag from the firstsub:I could not convince clang to produce this assembly, even using
__builtin_sub_overflow:https://godbolt.org/z/6vPWjx1zv
I could only produce this assembly by writing the LLVM IR manually:
https://godbolt.org/z/hvjEEsG9r
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