[PATCH] D135625: [instcombine] (extelt (inselt Vec, Value, Index), Index) -> Value

[Daniel Sanders via Phabricator via llvm-commits]

dsanders created this revision.
dsanders added a reviewer: reames.
Herald added subscribers: arphaman, hiraditya.
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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 at 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


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D135625

Files:
  llvm/lib/Transforms/InstCombine/InstCombineVectorOps.cpp
  llvm/test/Transforms/InstCombine/vector-inseltextelt.ll


Index: llvm/test/Transforms/InstCombine/vector-inseltextelt.ll
===================================================================
--- /dev/null
+++ 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
+}
Index: llvm/lib/Transforms/InstCombine/InstCombineVectorOps.cpp
===================================================================
--- llvm/lib/Transforms/InstCombine/InstCombineVectorOps.cpp
+++ llvm/lib/Transforms/InstCombine/InstCombineVectorOps.cpp
@@ -474,6 +474,11 @@
 
   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.


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