https://gcc.gnu.org/bugzilla/show_bug.cgi?id=71753
Bug ID: 71753
Summary: Clamp function does not work with O3 optimization
Product: gcc
Version: 7.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c++
Assignee: unassigned at gcc dot gnu.org
Reporter: lukasz.spintzyk at displaylink dot com
Target Milestone: ---
Created attachment 38830
--> https://gcc.gnu.org/bugzilla/attachment.cgi?id=38830&action=edit
Code that reproduces the issue
Hi,
The issue is that with O3 flag the fast_clamp function returns wrong value.
I have read your warning about compiler flags but i decided to submit that.
Sorry for that it if this is invalid.
I have tested the code with fwrapv flag and indeed it fix the problem so
probably the issue is related to signed arithmetic overflow optimizations.
I am submitting this as it can point real issue, on clang it is working and it
is producing very similar output(see below).
I have tested this against various gcc versions and problem seems to be
introduced between 4.9.3 and 5.1. Also reproducible on gcc 7.0
How to compile:
g++ -std=c++11 -O3 testClamp.cpp
When error happens running app should print following error:
> ./a.out
Fatal error, clamp of 260 gives 4 instead of 255
This issue is not reproducible on clang with the same optimizations enabled.
I was comparing its assembly output and functions differs only with one
instruction (movzbl %dil,%eax). Maybe this issue can be fixed without disabling
some of the optimizations with fwrapv flag.
gcc assebly:
0000000000000020 <_Z10fast_clampi>:
20: 8d 87 00 ff ff 7f lea 0x7fffff00(%rdi),%eax
26: c1 f8 1f sar $0x1f,%eax
29: 09 f8 or %edi,%eax
2b: c1 ff 1f sar $0x1f,%edi
2e: f7 d7 not %edi
30: 21 f8 and %edi,%eax
32: c3 retq
0000000000000020 <_Z10fast_clampi>:
20: 8d 87 00 ff ff 7f lea 0x7fffff00(%rdi),%eax
26: c1 f8 1f sar $0x1f,%eax
29: 09 f8 or %edi,%eax
2b: c1 ff 1f sar $0x1f,%edi
2e: f7 d7 not %edi
30: 21 c7 and %eax,%edi
32: 40 0f b6 c7 movzbl %dil,%eax
36: c3 retq
