From 288604ed0c2dd9a66093c525c7525bb2e7ed91fc Mon Sep 17 00:00:00 2001
From: Paul Redmond <paul.redmond@intel.com>
Date: Tue, 12 Feb 2013 15:21:21 +0000
Subject: [PATCH] PR14562 - Truncation of left shift became undef

DAGCombiner::ReduceLoadWidth was converting (trunc i32 (shl i64 v, 32))
into (shl i32 v, 32) into undef. To prevent this, check the shift count
against the final result size.

Patch by: Kevin Schoedel
Reviewed by: Nadav Rotem

llvm-svn: 174972
---
 llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp | 11 +++++++++--
 llvm/test/CodeGen/X86/pr14562.ll              | 15 +++++++++++++++
 2 files changed, 24 insertions(+), 2 deletions(-)
 create mode 100644 llvm/test/CodeGen/X86/pr14562.ll

diff --git a/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp b/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
index 5b83149..67fa39d 100644
--- a/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
+++ b/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
@@ -5163,8 +5163,15 @@ SDValue DAGCombiner::ReduceLoadWidth(SDNode *N) {
     EVT ShImmTy = getShiftAmountTy(Result.getValueType());
     if (!isUIntN(ShImmTy.getSizeInBits(), ShLeftAmt))
       ShImmTy = VT;
-    Result = DAG.getNode(ISD::SHL, N0.getDebugLoc(), VT,
-                         Result, DAG.getConstant(ShLeftAmt, ShImmTy));
+    // If the shift amount is as large as the result size (but, presumably,
+    // no larger than the source) then the useful bits of the result are
+    // zero; we can't simply return the shortened shift, because the result
+    // of that operation is undefined.
+    if (ShLeftAmt >= VT.getSizeInBits())
+      Result = DAG.getConstant(0, VT);
+    else
+      Result = DAG.getNode(ISD::SHL, N0.getDebugLoc(), VT,
+                          Result, DAG.getConstant(ShLeftAmt, ShImmTy));
   }
 
   // Return the new loaded value.
diff --git a/llvm/test/CodeGen/X86/pr14562.ll b/llvm/test/CodeGen/X86/pr14562.ll
new file mode 100644
index 0000000..2df64b3
--- /dev/null
+++ b/llvm/test/CodeGen/X86/pr14562.ll
@@ -0,0 +1,15 @@
+; RUN: llc < %s -march=x86 | FileCheck %s
+
+@temp1 = global i64 -77129852189294865, align 8
+
+define void @foo() nounwind {
+  %x = load i64* @temp1, align 8
+  %s = shl i64 %x, 32
+  %t = trunc i64 %s to i32
+  %z = zext i32 %t to i64
+  store i64 %z, i64* @temp1, align 8
+; CHECK: movl $0, temp1+4
+; CHECK: movl $0, temp1
+  ret void
+}
+
-- 
2.7.4