llvm/llvm-project
Vedi su GitHub[x86][armv8] Failure to use flag in unsigned underflow compare idiom
Open
#161.036 aperta il 28 set 2025
good first issuellvm:SelectionDAGmissed-optimization
Metriche repository
- Star
- (26.378 star)
- Metriche merge PR
- (Merge medio 1g 2h) (1000 PR mergiate in 30 g)
Descrizione
It is a common idiom to check for unsigned underflow by doing a greater than compare between the difference and minuend:
#include <stdint.h>
uint64_t subIfNoUnderflow_clang(uint64_t a, uint64_t b) {
uint64_t diff = a - b;
return (diff > a)? a : diff;
}
Instead of using the carry flag, which is set by the subtraction, clang emits a compare instruction:
https://godbolt.org/z/ex5554M6P
subIfNoUnderflow_clang:
mov rax, rdi
sub rax, rsi
cmp rax, rdi
cmovae rax, rdi
ret
Ideally it should produce:
subIfNoUnderflow_ideal:
mov rax, rdi
sub rax, rsi
cmovb rax, rdi
ret
Related issues
#53432 #62696 #73847