This is the mail archive of the
gcc-bugs@gcc.gnu.org
mailing list for the GCC project.
[Bug tree-optimization/69984] [4.9/5/6] Signed comparison instruction emitted for unsigned variable comparison
- From: "edmar at freescale dot com" <gcc-bugzilla at gcc dot gnu dot org>
- To: gcc-bugs at gcc dot gnu dot org
- Date: Mon, 29 Feb 2016 18:16:21 +0000
- Subject: [Bug tree-optimization/69984] [4.9/5/6] Signed comparison instruction emitted for unsigned variable comparison
- Auto-submitted: auto-generated
- References: <bug-69984-4 at http dot gcc dot gnu dot org/bugzilla/>
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=69984
--- Comment #6 from Edmar Wienskoski <edmar at freescale dot com> ---
Hummm, You are almost convincing me, one last question,
be patient with me.
As Andrew posted:
C = A * B
should be equivalent to:
C = (unsigned long)( ((int)A) * ((int)B) )
The variables are promoted *before* the multiplication.
How come (int)65535 multiplied by itself is undefined behavior ?
Or (unsigned short) 0x7FFF multiplied by itself, would not be undefined
behavior as well ?
In fact, re-writing the code as above, will generate unsigned comparison.
So, why "C = A * B" is not doing the same ?