https://gcc.gnu.org/bugzilla/show_bug.cgi?id=96512
Bug ID: 96512
Summary: wrong code generated with avx512 intrinsics in some
cases
Product: gcc
Version: 8.3.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: tree-optimization
Assignee: unassigned at gcc dot gnu.org
Reporter: nathanael.schaeffer at gmail dot com
Target Milestone: ---
Created attachment 49013
--> https://gcc.gnu.org/bugzilla/attachment.cgi?id=49013&action=edit
bug demonstrator
with gcc 8.3.0 (and also possibly some other versions), I found a nasty bug
that messes up the results of my calculations.
I worked hard to produce a simple reproduced, which is attached.
With gcc 8.3.0, compiling with
gcc -O1 -g -D_GCC_VEC_=1 -march=skylake-avx512 bug_gcc_avx512.c
running ./a.out leads to a wrong result, displayed like so:
ERROR :: 0.874347 == 0
Examining the generated assembly, I suspect this instruction to be wrong:
0x0000000000401186 <+100>: vbroadcastsd zmm0,QWORD PTR [r8*8+0x1]
because r8 is aligned, and the 0x1 offset does not seem right...
When compiling with -march=skylake the problem goes away.
When using "alloca" instead of variable length array, the problem goes away
When inserting a printf, the problem goes away.
See in the attached source file. The lines commented with "NO BUG" make the bug
go away.
This has been a nightmare to spot, as it does not happen on all compiler
versions. I hope somebody can reproduce it and fix it...
Note that the assembly generated on godbolt does not seem to have this issue...
