[instcombine] (extelt (inselt Vec, Value, Index), Index) -> Value

When Index is variable but still trivially known to be equal we can use Value
from before the insertion, possibly eliminating the vector.

Reverts a functional change from:
Author: Philip Reames <listmail@philipreames.com>
Date:   Wed Dec 8 12:21:10 2021 -0800

    [instcombine] A couple style tweaks to visitExtractElementInst [nfc]

Thanks to Michele Scandale for identifying the bug

Differential Revision: https://reviews.llvm.org/D135625

diff --git a/llvm/lib/Transforms/InstCombine/InstCombineVectorOps.cpp b/llvm/lib/Transforms/InstCombine/InstCombineVectorOps.cpp
index 40acc24..9bb12cb 100644 (file)
--- a/llvm/lib/Transforms/InstCombine/InstCombineVectorOps.cpp
+++ b/llvm/lib/Transforms/InstCombine/InstCombineVectorOps.cpp
@@ -474,6 +474,11 @@ Instruction *InstCombinerImpl::visitExtractElementInst(ExtractElementInst &EI) {
 
   if (auto *I = dyn_cast<Instruction>(SrcVec)) {
     if (auto *IE = dyn_cast<InsertElementInst>(I)) {
+      // If the possibly-variable indices are trivially known to be equal
+      // (because they are the same operand) then use the value that was
+      // inserted directly.
+      if (IE->getOperand(2) == Index)
+        return replaceInstUsesWith(EI, IE->getOperand(1));
       // instsimplify already handled the case where the indices are constants
       // and equal by value, if both are constants, they must not be the same
       // value, extract from the pre-inserted value instead.

diff --git a/llvm/test/Transforms/InstCombine/vector-inseltextelt.ll b/llvm/test/Transforms/InstCombine/vector-inseltextelt.ll
new file mode 100644 (file)
index 0000000..b0a6a4e
--- /dev/null
+++ b/llvm/test/Transforms/InstCombine/vector-inseltextelt.ll
@@ -0,0 +1,23 @@
+; NOTE: Assertions have been autogenerated by utils/update_test_checks.py
+; RUN: opt < %s -passes=instcombine -S | FileCheck %s
+
+; extracting a just-inserted element should yield the original value
+define float @constant_index(<4 x float> %x, float %val) {
+; CHECK-LABEL: @constant_index(
+; CHECK-NEXT:    ret float [[VAL:%.*]]
+;
+  %ins = insertelement <4 x float> %x, float %val, i32 1
+  %ext = extractelement <4 x float> %ins, i32 1
+  ret float %ext
+}
+
+; extracting a just-inserted element should yield the original value even if
+; the index is dynamic
+define float @dynamic_index(<4 x float> %x, float %val, i32 %idx) {
+; CHECK-LABEL: @dynamic_index(
+; CHECK-NEXT:    ret float [[VAL:%.*]]
+;
+  %ins = insertelement <4 x float> %x, float %val, i32 %idx
+  %ext = extractelement <4 x float> %ins, i32 %idx
+  ret float %ext
+}