After upgrading to 4.4.1 one of my unit tests started failing reproducibly.
I tried to find the part of -O3 (compared to -O2) that caused the failure and I
was able to "fix" the error by changing "optimize >= 3" to "optimize >= 4" in
tree-ssa-loop.c lines 509 and 550. That's the minimal change to make it work
again.
Next I tried to reduce my code to a simple testcase but that is really hard. I
will attach the smallest file I was able to reduce. But in that case it also
fails at -O3 (with my change above -O4 was necessary to make it fail), -O2 and
-O1. -O0 still doesn't fail.
Any further simplification of the code made the bug go away.
The code creates an array of floats which an index vector (starting from {0,
1}) gathers values from into a float vector class. Then the code verifies that
the correct values were read.
The error I was able to identify in the assembly is that the multiplication at
line 93 ({0, 1} * uint_v(1)) is carried out as {0, 1} * {2, 3} instead of {0,
1} * {1, 1}.
--
Summary: 4.4.1 regression: miscompiles with -03 (seemingly
related to attribute may_alias)
Product: gcc
Version: 4.4.1
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: middle-end
AssignedTo: unassigned at gcc dot gnu dot org
ReportedBy: kretz at kde dot org
GCC build triplet: x86_64-unknown-linux-gnu
GCC host triplet: x86_64-unknown-linux-gnu
GCC target triplet: x86_64-unknown-linux-gnu
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=40924
