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

[drcut]

Summary

This patch optimizes dead store which cannot be optimized due to the previous capture tracking which does not concern Constant Expression

[nikic]

This code is known to be incorrect, so rejecting its extension. See https://github.com/llvm/llvm-project/issues/54002, as well as D120371 for the proper way to handle this.

llvm/lib/Analysis/CaptureTracking.cpp

if (auto *CPN = dyn_cast<ConstantPointerNull>(I->getOperand(OtherIdx))) {
       }
     }
     // Comparison against value stored in global variable. Given the pointer
     // does not escape, its value cannot be guessed and stored separately in a
     // global variable.
     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;
   }
   default:
     // Something else - be conservative and say it is captured.
     return UseCaptureKind::MAY_CAPTURE;
   }

llvm/test/Transforms/DeadStoreElimination/const-expr.ll

+; NOTE: Assertions have been autogenerated by utils/update_test_checks.py
+; RUN: opt -passes=dse -S < %s | FileCheck %s
+@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)