The vectorizer doesn't appear to handle packed structures correctly and
ultimately generates a vector store to an unaligned address which causes a
segfault.
This was originally reported against a 4.4 variant, but investigation shows the
bug is merely latent on the head of the trunk and can be exposed by reverting
this change:
2009-03-29 Richard Guenther <rguent...@suse.de>
* tree-ssa-forwprop.c (forward_propagate_addr_expr_1): Properly
propagate addresses of array references.
[ The 2009-03-29 patch changes the end condition of the loop which ultimately
confuses the vectorizer into being unable to determine the number of iterations
in the loop. That's clearly a weakness in the vectorizer as the loop bounds
are easily shown to be compile-time constants. ]
Compile the attached testcase with -O3 and search the resulting assembly for
these two key insns:
leaq 2(%rax,%rdi,4), %rdi
This computes the base address for palette->ents which is at offset 2 in the
Palette structure (which is packed). This insn appears in the vectorized loop
pre-header. It's later used by:
movdqa %xmm0, (%rdi,%rsi)
The faulting vector insn.
Note carefully this isn't a case where peeling iterations can be used to
generate suitable alignment -- the packed nature of the structure results in
all the elt members being unaligned.
ISTM that vector_alignment_reachable_p is the place to fix this, but I'm not
very familiar with the vectorizer and thus may be off base.
--
Summary: vectorizer created unaligned vector insns
Product: gcc
Version: unknown
Status: UNCONFIRMED
Keywords: wrong-code
Severity: normal
Priority: P3
Component: tree-optimization
AssignedTo: unassigned at gcc dot gnu dot org
ReportedBy: law at redhat dot com
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=42652
