https://gcc.gnu.org/bugzilla/show_bug.cgi?id=81450
Bug ID: 81450
Summary: Typedef with assume aligned builtin yields
segmentation fault in nested loop
Product: gcc
Version: 6.3.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: ipa
Assignee: unassigned at gcc dot gnu.org
Reporter: philipp.kopp at tum dot de
CC: marxin at gcc dot gnu.org
Target Milestone: ---
Created attachment 41762
--> https://gcc.gnu.org/bugzilla/attachment.cgi?id=41762&action=edit
Small example with the working and the failing version
Hi,
I found that the assume aligned built-in gives a segmentation fault in nested
loops when using it with a typedef (or the C++ 11 using keyword). I attached a
small example code. I am using Ubuntu 17.04 with the standard gcc 6.3.0 from
the Ubuntu archives. I am compiling with gcc -O3 -g. So SSE vectorization is
enabled, without that the code runs fine.
The core part where execution fails is the following:
> typedef double __attribute__((aligned (32))) AlignedDouble;
> const size_t size = 17;
> double alpha = 1.0 / 3.0;
> // uses posix_memalign (see full testcase)
> AlignedDouble* A = aligned_doubles( size * size );
> for( size_t i = 0; i < size; ++i )
> {
> printf( "i = %lu\n", i );
> for( size_t j = 0; j < size; ++j )
> {
> A[j + i * size] += alpha;
> }
> }
I checked the disassembly and found that the loop over j runs once cpompletely
and fails the second time, as a temporary pointer to A is incremented by i *
size, which will yield an unaligned pointer if size % 2 != 0 (or 4 with avx2).
However, if the alignment is directly used in the definition of A, without
using a typedef, the result is different. Looking at the output of
-fopt-info-loop the loop is peeled for alignment in this case.
With typedef:
> gcc -O3 -g main.cpp -o main -fopt-info-loop
> main.cpp:40:26: note: loop vectorized
> main.cpp:40:26: note: loop turned into non-loop; it never loops
> main.cpp:15:5: note: loop turned into non-loop; it never loops.
> main.cpp:15:5: note: loop with 8 iterations completely unrolled
Without typedef:
> gcc -O3 -g main.cpp -o main -fopt-info-loop
> main.cpp:30:26: note: loop vectorized
> main.cpp:30:26: note: loop peeled for vectorization to enhance alignment
> main.cpp:30:26: note: loop turned into non-loop; it never loops
> main.cpp:15:5: note: loop turned into non-loop; it never loops.
> main.cpp:15:5: note: loop with 9 iterations completely unrolled
> main.cpp:15:5: note: loop turned into non-loop; it never loops
In the attached test case you find for both scenarios the code, the binary, an
objdump, as well as outputs from -fopt-info-loop and -fopt-info-all. In the
objdum I marked the increment of A as well as the instruction where the
segmentation fault happens.
Please let me know if more info is needed. Thanks a lot for your time!
Best wishes,
Philipp Kopp
