[SLP] respect target register width for GEP vectorization (PR43578)

We failed to account for the target register width (max vector factor)
when vectorizing starting from GEPs. This causes vectorization to
proceed to obviously illegal widths as in:
https://bugs.llvm.org/show_bug.cgi?id=43578

For x86, this also means that SLP can produce rogue AVX or AVX512
code even when the user specifies a narrower vector width.

The AArch64 test in ext-trunc.ll appears to be better using the
narrower width. I'm not exactly sure what getelementptr.ll is trying
to do, but it's testing with "-slp-threshold=-18", so I'm not worried
about those diffs. The x86 test is an over-reduction from SPEC h264;
this patch appears to restore the perf loss caused by SLP when using
-march=haswell.

Differential Revision: https://reviews.llvm.org/D68667

llvm-svn: 374183

1 files changed, 10 insertions, 4 deletions

diff --git a/llvm/lib/Transforms/Vectorize/SLPVectorizer.cpp b/llvm/lib/Transforms/Vectorize/SLPVectorizer.cpp
index 99428c6c5de..75b4718392d 100644
--- a/llvm/lib/Transforms/Vectorize/SLPVectorizer.cpp
+++ b/llvm/lib/Transforms/Vectorize/SLPVectorizer.cpp
@@ -6981,10 +6981,16 @@ bool SLPVectorizerPass::vectorizeGEPIndices(BasicBlock *BB, BoUpSLP &R) {
     LLVM_DEBUG(dbgs() << "SLP: Analyzing a getelementptr list of length "
                       << Entry.second.size() << ".\n");
 
-    // We process the getelementptr list in chunks of 16 (like we do for
-    // stores) to minimize compile-time.
-    for (unsigned BI = 0, BE = Entry.second.size(); BI < BE; BI += 16) {
-      auto Len = std::min<unsigned>(BE - BI, 16);
+    // Process the GEP list in chunks suitable for the target's supported
+    // vector size. If a vector register can't hold 1 element, we are done.
+    unsigned MaxVecRegSize = R.getMaxVecRegSize();
+    unsigned EltSize = R.getVectorElementSize(Entry.second[0]);
+    if (MaxVecRegSize < EltSize)
+      continue;
+
+    unsigned MaxElts = MaxVecRegSize / EltSize;
+    for (unsigned BI = 0, BE = Entry.second.size(); BI < BE; BI += MaxElts) {
+      auto Len = std::min<unsigned>(BE - BI, MaxElts);
       auto GEPList = makeArrayRef(&Entry.second[BI], Len);
 
       // Initialize a set a candidate getelementptrs. Note that we use a