GlobalOpt: If we have an inbounds GEP from a ConstantAggregateZero global that we just determined to be constant, replace all loads from it with a zero value.

llvm-svn: 153576

2 files changed, 17 insertions, 0 deletions

diff --git a/llvm/lib/Transforms/IPO/GlobalOpt.cpp b/llvm/lib/Transforms/IPO/GlobalOpt.cpp
index a32e5509545..1522aa408b6 100644
--- a/llvm/lib/Transforms/IPO/GlobalOpt.cpp
+++ b/llvm/lib/Transforms/IPO/GlobalOpt.cpp
@@ -341,6 +341,12 @@ static bool CleanupConstantGlobalUsers(Value *V, Constant *Init,
           dyn_cast_or_null<ConstantExpr>(ConstantFoldInstruction(GEP, TD, TLI));
         if (Init && CE && CE->getOpcode() == Instruction::GetElementPtr)
           SubInit = ConstantFoldLoadThroughGEPConstantExpr(Init, CE);
+
+        // If the initializer is an all-null value and we have an inbounds GEP,
+        // we already know what the result of any load from that GEP is.
+        // TODO: Handle splats.
+        if (Init && isa<ConstantAggregateZero>(Init) && GEP->isInBounds())
+          SubInit = Constant::getNullValue(GEP->getType()->getElementType());
       }
       Changed |= CleanupConstantGlobalUsers(GEP, SubInit, TD, TLI);
 
diff --git a/llvm/test/Transforms/GlobalOpt/zeroinitializer-gep-load.ll b/llvm/test/Transforms/GlobalOpt/zeroinitializer-gep-load.ll
new file mode 100644
index 00000000000..d613601e8d7
--- /dev/null
+++ b/llvm/test/Transforms/GlobalOpt/zeroinitializer-gep-load.ll
@@ -0,0 +1,11 @@
+; RUN: opt < %s -S -globalopt | FileCheck %s
+
+@zero = internal global [10 x i32] zeroinitializer
+
+define i32 @test1(i64 %idx) nounwind {
+  %arrayidx = getelementptr inbounds [10 x i32]* @zero, i64 0, i64 %idx
+  %l = load i32* %arrayidx
+  ret i32 %l
+; CHECK: @test1
+; CHECK: ret i32 0
+}