Re: optimizer discards sign information

[LIU Hao <lh_mouse@126.com>]

[-- Attachment #1.1: Type: text/plain, Size: 839 bytes --]

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


[-- Attachment #2: OpenPGP digital signature --]
[-- Type: application/pgp-signature, Size: 840 bytes --]