[PATCH] D130672: [llvm] Extend the CaptureTracking to handle constantExprs that read global variables

[Ruobing Han via Phabricator via llvm-commits]

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This patch can optimize dead store which cannot be optimized due to the conservative capture tracking which does not concern Constant Expression


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D130672

Files:
  llvm/lib/Analysis/CaptureTracking.cpp
  llvm/test/Transforms/DeadStoreElimination/const-expr.ll


Index: llvm/test/Transforms/DeadStoreElimination/const-expr.ll
===================================================================
--- /dev/null
+++ llvm/test/Transforms/DeadStoreElimination/const-expr.ll
@@ -0,0 +1,18 @@
+; NOTE: Assertions have been autogenerated by utils/update_test_checks.py
+; RUN: opt -passes=dse -S < %s | FileCheck %s
+ at global = external constant [4 x i8]
+
+define internal void @f() {
+; CHECK-LABEL: @f(
+; CHECK-NEXT:    [[TMP1:%.*]] = call noalias i8* @_Znwm(i64 32)
+; CHECK-NEXT:    [[TMP2:%.*]] = icmp ugt i8* [[TMP1]], getelementptr inbounds ([4 x i8], [4 x i8]* @global, i64 0, i64 0)
+; CHECK-NEXT:    ret void
+;
+  %tmp1 = call noalias i8* @_Znwm(i64 32)
+  %tmp2 = icmp ugt i8* %tmp1, getelementptr inbounds ([4 x i8], [4 x i8]* @global, i64 0, i64 0)
+  %tmp3 = getelementptr inbounds i8, i8* %tmp1, i64 3
+  store i8 0, i8* %tmp3
+  ret void
+}
+
+declare i8* @_Znwm(i64)
Index: llvm/lib/Analysis/CaptureTracking.cpp
===================================================================
--- llvm/lib/Analysis/CaptureTracking.cpp
+++ llvm/lib/Analysis/CaptureTracking.cpp
@@ -424,6 +424,11 @@
     auto *LI = dyn_cast<LoadInst>(I->getOperand(OtherIdx));
     if (LI && isa<GlobalVariable>(LI->getPointerOperand()))
       return UseCaptureKind::NO_CAPTURE;
+    if (auto *CE = dyn_cast<ConstantExpr>(I->getOperand(OtherIdx))) {
+      if (CE->getOpcode() == Instruction::GetElementPtr &&
+          isa<GlobalVariable>(CE->getOperand(0)))
+        return UseCaptureKind::NO_CAPTURE;
+    }
     // Otherwise, be conservative. There are crazy ways to capture pointers
     // using comparisons.
     return UseCaptureKind::MAY_CAPTURE;


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