llvm/llvm-project
Auf GitHub ansehen[x86][armv8] Failure to use flag in unsigned underflow compare idiom
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#161.036 geöffnet am 28. Sept. 2025
good first issuellvm:SelectionDAGmissed-optimization
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Beschreibung
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