https://llvm.org/bugs/show_bug.cgi?id=28981
Bug ID: 28981
Summary: Missed forced vectorization due to
llvm.mem.parallel_loop_access getting lost
Product: tools
Version: trunk
Hardware: All
OS: Linux
Status: NEW
Severity: normal
Priority: P
Component: opt
Assignee: unassignedb...@nondot.org
Reporter: dorit.nuz...@intel.com
CC: llvm-bugs@lists.llvm.org
Classification: Unclassified
Created attachment 16953
--> https://llvm.org/bugs/attachment.cgi?id=16953&action=edit
Example loop with #pragma clang loop vectorize(assume_safety)
The following loop does not get vectorized because we lose some of the
information that is implied by "#pragma clang loop vectorize(assume_safety)":
void test(Complex *out, long size)
{
long m = size / 2;
#pragma clang loop vectorize(assume_safety)
for (long offset = 0; offset < m; ++offset) {
Complex t0 = out[offset];
Complex t1 = out[offset+m];
out[offset] = t0 + t1;
out[offset+m] = t0 - t1;
}
}
clang++ -S force-vect-test1.cpp -g -m64 -Ofast -funroll-loops -static
-mfpmath=sse -march=core-avx2 -Rpass-analysis=loop-vectorize
force-vect-test1.cpp:29:5: remark: loop not vectorized: cannot check memory
dependencies at runtime [-Rpass-analysis]
for (long offset = 0; offset < m; ++offset) {
^
force-vect-test1.cpp:29:5: warning: loop not vectorized: failed explicitly
specified loop vectorization [-Wpass-failed]
By the time this loop reaches the vectorizer, some of the memory operations in
the loop don't have the llvm.mem.parallel_loop_access metadata, because:
1) clang does not annotate the memcpy that it creates in the loop with the
llvm.mem.parallel_loop_access metadata (this is PR28980)
2) Even if we annotate the memcpy manually, InstCombine later replaces it with
a ld/st and does not copy over the llvm.mem.parallel_loop_access metadata (this
is D23499)
3) later SROA does not preserve the llvm.mem.parallel_loop_access metadata
during its transformations (will upload a proposed patch shortly)
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