[PATCH] GVN: Use proper members for accessing into GEPs

[David Majnemer]

Hi nicholas,

GVN wants to take the following global value:
@main.c = private unnamed_addr constant { [2 x i8], i32, i8, [3 x i8] } { [2 x i8] zeroinitializer, i32 0, i8 1, [3 x i8] undef }, align 4

and index into offset '8' to extract the i8 holding the value 1.

in GetMemInstValueForLoad, we construct the following ConstantExpr:
i8* getelementptr ({ [2 x i8], i32, i8, [3 x i8] }* @main.c, i32 0, i32 0, i64 8)

This GEP was formed by way of a ConstantExpr::getBitCast which wanted to turn @main.c into an i8*
The constant folding logic in FoldBitCast decided to reuse the first
field (which was already an i8*) and form a GEP instead of a bitcast
instruction.

GetMemInstValueForLoad proceeds to call
ConstantFoldLoadThroughGEPConstantExpr which walks thru the GEP and
tries to access the third element of a ConstantAggregateZero which will
always give you zero.

http://llvm-reviews.chandlerc.com/D2060

Files:
  lib/Transforms/Scalar/GVN.cpp
  test/Transforms/GVN/pr17732.ll

Index: lib/Transforms/Scalar/GVN.cpp
===================================================================
--- lib/Transforms/Scalar/GVN.cpp
+++ lib/Transforms/Scalar/GVN.cpp
@@ -1256,6 +1256,11 @@
   ConstantInt::get(Type::getInt64Ty(Src->getContext()), (unsigned)Offset);
   Src = ConstantExpr::getGetElementPtr(Src, OffsetCst);
   Src = ConstantExpr::getBitCast(Src, PointerType::getUnqual(LoadTy));
+  if (ConstantExpr *SrcCE = dyn_cast<ConstantExpr>(Src)) {
+    Src = ConstantFoldConstantExpression(SrcCE, &TD);
+    if (!Src)
+      return 0;
+  }
   return ConstantFoldLoadFromConstPtr(Src, &TD);
 }
 
Index: test/Transforms/GVN/pr17732.ll
===================================================================
--- /dev/null
+++ test/Transforms/GVN/pr17732.ll
@@ -0,0 +1,21 @@
+; RUN: opt -gvn -S -o - < %s | FileCheck %s
+
+target datalayout = "e-p:64:64:64-i1:8:8-i8:8:8-i16:16:16-i32:32:32-i64:64:64-f32:32:32-f64:64:64-v64:64:64-v128:128:128-a0:0:64-s0:64:64-f80:128:128-n8:16:32:64-S128"
+target triple = "x86_64-unknown-linux-gnu"
+
+%struct.S2 = type { [2 x i8], i32, i8 }
+
+ at main.c = private unnamed_addr constant { [2 x i8], i32, i8, [3 x i8] } { [2 x i8] zeroinitializer, i32 0, i8 1, [3 x i8] undef }, align 4
+ at b = common global %struct.S2 zeroinitializer, align 4
+
+define i32 @main() {
+entry:
+  tail call void @llvm.memcpy.p0i8.p0i8.i64(i8* getelementptr inbounds (%struct.S2* @b, i64 0, i32 0, i64 0), i8* getelementptr inbounds ({ [2 x i8], i32, i8, [3 x i8] }* @main.c, i64 0, i32 0, i64 0), i64 12, i32 4, i1 false)
+  %0 = load i8* getelementptr inbounds (%struct.S2* @b, i64 0, i32 2), align 4
+  %conv = sext i8 %0 to i32
+  ret i32 %conv
+; CHECK-LABEL: define i32 @main(
+; CHECK: ret i32 1
+}
+
+declare void @llvm.memcpy.p0i8.p0i8.i64(i8* nocapture, i8* nocapture readonly, i64, i32, i1)
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