From: Benjamin Kramer <benny.kra@googlemail.com>
Date: Wed, 28 Mar 2012 14:50:09 +0000 (+0000)
Subject: GlobalOpt: If we have an inbounds GEP from a ConstantAggregateZero global that we... 
X-Git-Url: http://demsky.eecs.uci.edu/git/?a=commitdiff_plain;h=c1ea16ec4339a9746f436c6f2f5b08197f309343;p=oota-llvm.git

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.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@153576 91177308-0d34-0410-b5e6-96231b3b80d8
---

diff --git a/lib/Transforms/IPO/GlobalOpt.cpp b/lib/Transforms/IPO/GlobalOpt.cpp
index a32e5509545..1522aa408b6 100644
--- a/lib/Transforms/IPO/GlobalOpt.cpp
+++ b/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/test/Transforms/GlobalOpt/zeroinitializer-gep-load.ll b/test/Transforms/GlobalOpt/zeroinitializer-gep-load.ll
new file mode 100644
index 00000000000..d613601e8d7
--- /dev/null
+++ b/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
+}