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From: <stefan@franke.ms>
To: <gcc-help@gcc.gnu.org>
Subject: optimizer discards sign information
Date: Wed, 10 Apr 2024 10:52:42 +0200
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Hi all,

I just stumbled over an issue, which is present in almost all gcc =
versions.
I worked around using inline assembly=85
Maybe gcc behaves correct and I am wrong? Here is the code:

https://godbolt.org/z/cW8jcdh56

typedef unsigned long long int u64;
typedef unsigned int u32;
typedef unsigned short u16;

u64 foo(u16 a, u16 b) {
=A0 =A0 u32 x =3D a * b;
=A0 =A0 u64 r =3D x;
=A0 =A0 return r;
}

And on gcc 13.2 x86.64 you get

foo:
=A0 =A0 =A0 =A0 movzx =A0 esi, si
=A0 =A0 =A0 =A0 movzx =A0 edi, di
=A0 =A0 =A0 =A0 imul =A0 =A0edi, esi
=A0 =A0 =A0 =A0 movsx =A0 rax, edi
=A0 =A0 =A0 =A0 ret


There is a sign extension! The optimizer step discards the information

	=A0x_6 =3D (u32) _3;

and uses _3 directly instead, which is signed.

Am I wrong or is it gcc?

Since this is a tree pass, all targets are affected. And it=92s already
present in gcc-6.5

Best regards

Stefan=20