[PATCH] D104867: [InstSimplify] fold extractelement of splat even with variable index

[Sanjay Patel via Phabricator via llvm-commits]

spatel created this revision.
spatel added reviewers: CarolineConcatto, sdesmalen, frasercrmck, RKSimon.
Herald added subscribers: arphaman, hiraditya, mcrosier.
spatel requested review of this revision.
Herald added a project: LLVM.

We unnecessarily restricted a fold of a splat to a constant vector.

We overlooked this fold in D102404 <https://reviews.llvm.org/D102404> and earlier patches, but the fixed vector variant is shown in:
https://llvm.org/PR50817

Alive2 agrees on that:
https://alive2.llvm.org/ce/z/HpijPC

I don't think Alive2 understands scalable vectors, but the same logic applies IIUC.


https://reviews.llvm.org/D104867

Files:
  llvm/lib/Analysis/InstructionSimplify.cpp
  llvm/test/Transforms/InstSimplify/extract-element.ll
  llvm/test/Transforms/InstSimplify/vscale-inseltpoison.ll
  llvm/test/Transforms/InstSimplify/vscale.ll


Index: llvm/test/Transforms/InstSimplify/vscale.ll
===================================================================
--- llvm/test/Transforms/InstSimplify/vscale.ll
+++ llvm/test/Transforms/InstSimplify/vscale.ll
@@ -210,10 +210,7 @@
 
 define i32 @extractelement_splat_variable_index(i32 %v, i32 %idx) {
 ; CHECK-LABEL: @extractelement_splat_variable_index(
-; CHECK-NEXT:    [[IN:%.*]] = insertelement <vscale x 4 x i32> undef, i32 [[V:%.*]], i32 0
-; CHECK-NEXT:    [[SPLAT:%.*]] = shufflevector <vscale x 4 x i32> [[IN]], <vscale x 4 x i32> undef, <vscale x 4 x i32> zeroinitializer
-; CHECK-NEXT:    [[R:%.*]] = extractelement <vscale x 4 x i32> [[SPLAT]], i32 [[IDX:%.*]]
-; CHECK-NEXT:    ret i32 [[R]]
+; CHECK-NEXT:    ret i32 [[V:%.*]]
 ;
   %in = insertelement <vscale x 4 x i32> undef, i32 %v, i32 0
   %splat = shufflevector <vscale x 4 x i32> %in, <vscale x 4 x i32> undef, <vscale x 4 x i32> zeroinitializer
Index: llvm/test/Transforms/InstSimplify/vscale-inseltpoison.ll
===================================================================
--- llvm/test/Transforms/InstSimplify/vscale-inseltpoison.ll
+++ llvm/test/Transforms/InstSimplify/vscale-inseltpoison.ll
@@ -210,10 +210,7 @@
 
 define i32 @extractelement_splat_variable_index(i32 %v, i32 %idx) {
 ; CHECK-LABEL: @extractelement_splat_variable_index(
-; CHECK-NEXT:    [[IN:%.*]] = insertelement <vscale x 4 x i32> poison, i32 [[V:%.*]], i32 0
-; CHECK-NEXT:    [[SPLAT:%.*]] = shufflevector <vscale x 4 x i32> [[IN]], <vscale x 4 x i32> undef, <vscale x 4 x i32> zeroinitializer
-; CHECK-NEXT:    [[R:%.*]] = extractelement <vscale x 4 x i32> [[SPLAT]], i32 [[IDX:%.*]]
-; CHECK-NEXT:    ret i32 [[R]]
+; CHECK-NEXT:    ret i32 [[V:%.*]]
 ;
   %in = insertelement <vscale x 4 x i32> poison, i32 %v, i32 0
   %splat = shufflevector <vscale x 4 x i32> %in, <vscale x 4 x i32> undef, <vscale x 4 x i32> zeroinitializer
Index: llvm/test/Transforms/InstSimplify/extract-element.ll
===================================================================
--- llvm/test/Transforms/InstSimplify/extract-element.ll
+++ llvm/test/Transforms/InstSimplify/extract-element.ll
@@ -62,10 +62,7 @@
 
 define i32 @extractelement_splat_variable_index(i32 %v, i32 %index) {
 ; CHECK-LABEL: @extractelement_splat_variable_index(
-; CHECK-NEXT:    [[IN:%.*]] = insertelement <3 x i32> poison, i32 [[V:%.*]], i32 0
-; CHECK-NEXT:    [[SPLAT:%.*]] = shufflevector <3 x i32> [[IN]], <3 x i32> poison, <3 x i32> zeroinitializer
-; CHECK-NEXT:    [[R:%.*]] = extractelement <3 x i32> [[SPLAT]], i32 [[INDEX:%.*]]
-; CHECK-NEXT:    ret i32 [[R]]
+; CHECK-NEXT:    ret i32 [[V:%.*]]
 ;
   %in = insertelement <3 x i32> poison, i32 %v, i32 0
   %splat = shufflevector <3 x i32> %in, <3 x i32> poison, <3 x i32> zeroinitializer
Index: llvm/lib/Analysis/InstructionSimplify.cpp
===================================================================
--- llvm/lib/Analysis/InstructionSimplify.cpp
+++ llvm/lib/Analysis/InstructionSimplify.cpp
@@ -4528,10 +4528,6 @@
     if (auto *CIdx = dyn_cast<Constant>(Idx))
       return ConstantExpr::getExtractElement(CVec, CIdx);
 
-    // The index is not relevant if our vector is a splat.
-    if (auto *Splat = CVec->getSplatValue())
-      return Splat;
-
     if (Q.isUndefValue(Vec))
       return UndefValue::get(VecVTy->getElementType());
   }
@@ -4554,6 +4550,10 @@
         return Splat;
     if (Value *Elt = findScalarElement(Vec, IdxC->getZExtValue()))
       return Elt;
+  } else {
+    // The index is not relevant if our vector is a splat.
+    if (Value *Splat = getSplatValue(Vec))
+      return Splat;
   }
   return nullptr;
 }


-------------- next part --------------
A non-text attachment was scrubbed...
Name: D104867.354296.patch
Type: text/x-patch
Size: 3626 bytes
Desc: not available
URL: <http://lists.llvm.org/pipermail/llvm-commits/attachments/20210624/35975ecf/attachment.bin>