在 2024-04-10 17:19, Xi Ruoyao via Gcc-help 写道:
$ cc t.c -O2 -fsanitize=undefined $ ./a.out t.c:7:15: runtime error: signed integer overflow: 65535 * 65535 cannot be represented in type 'int' fffe0001
Undefined behavior is not a valid point, as it never happens. It's a real bug. There are many PRs on bugzilla.
The sign extension could have been eliminated completely:
movzx eax, si
movzx edi, di
imul eax, edi
ret
And here is a similar issue:
typedef unsigned long long int u64;
typedef unsigned int u32;
u64 foo(u64 a) {
return (u32) __builtin_ctzll(a);
}
which results in
xor eax, eax
rep bsf rax, rdi // effectively `tzcnt rax, rdi`
cdqe // unnecessary sign-extension
ret
--
Best regards,
LIU Hao
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