[RISCV] Match strided vector bases in RISCVGatherScatterLowering (#93972)

Currently we only match GEPs with a scalar base pointer, but a common
pattern that's emitted from the loop vectorizer is a strided vector base
plus some sort of scalar offset:

    %base = getelementptr i64, ptr %p, <vscale x 1 x i64> %step
    %gep = getelementptr i64, <vscale x 1 x ptr> %base, i64 %offset

This is common for accesses into a struct e.g. f[i].b below:

    struct F { int a; char b; };

    void foo(struct F *f) {
      for (int i = 0; i < 1024; i += 2) {
        f[i].a++;
        f[i].b++;
      }
    }

This patch handles this case in RISCVGatherScatterLowering by recursing
on the base pointer if it's a vector.

With this we can convert roughly 80% of the indexed loads and stores
emitted to strided loads and stores on SPEC CPU 2017, -O3
-march=rva22u64_v

1 files changed, 20 insertions, 1 deletions

diff --git a/llvm/lib/Target/RISCV/RISCVGatherScatterLowering.cpp b/llvm/lib/Target/RISCV/RISCVGatherScatterLowering.cpp
index f0bd25f..cff46e1 100644
--- a/llvm/lib/Target/RISCV/RISCVGatherScatterLowering.cpp
+++ b/llvm/lib/Target/RISCV/RISCVGatherScatterLowering.cpp
@@ -349,8 +349,27 @@ RISCVGatherScatterLowering::determineBaseAndStride(Instruction *Ptr,
 
   SmallVector<Value *, 2> Ops(GEP->operands());
 
+  // If the base pointer is a vector, check if it's strided.
+  Value *Base = GEP->getPointerOperand();
+  if (auto *BaseInst = dyn_cast<Instruction>(Base);
+      BaseInst && BaseInst->getType()->isVectorTy()) {
+    // If GEP's offset is scalar then we can add it to the base pointer's base.
+    auto IsScalar = [](Value *Idx) { return !Idx->getType()->isVectorTy(); };
+    if (all_of(GEP->indices(), IsScalar)) {
+      auto [BaseBase, Stride] = determineBaseAndStride(BaseInst, Builder);
+      if (BaseBase) {
+        Builder.SetInsertPoint(GEP);
+        SmallVector<Value *> Indices(GEP->indices());
+        Value *OffsetBase =
+            Builder.CreateGEP(GEP->getSourceElementType(), BaseBase, Indices,
+                              GEP->getName() + "offset", GEP->isInBounds());
+        return {OffsetBase, Stride};
+      }
+    }
+  }
+
   // Base pointer needs to be a scalar.
-  Value *ScalarBase = Ops[0];
+  Value *ScalarBase = Base;
   if (ScalarBase->getType()->isVectorTy()) {
     ScalarBase = getSplatValue(ScalarBase);
     if (!ScalarBase)