[PATCH] PR14562 - Truncation of left shift became undef

[Kevin Schoedel]

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.

http://llvm-reviews.chandlerc.com/D344

Files:
  lib/CodeGen/SelectionDAG/DAGCombiner.cpp
  test/CodeGen/X86/pr14562.ll

Index: lib/CodeGen/SelectionDAG/DAGCombiner.cpp
===================================================================
--- lib/CodeGen/SelectionDAG/DAGCombiner.cpp
+++ lib/CodeGen/SelectionDAG/DAGCombiner.cpp
@@ -5163,8 +5163,15 @@
     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.
Index: test/CodeGen/X86/pr14562.ll
===================================================================
--- /dev/null
+++ test/CodeGen/X86/pr14562.ll
@@ -0,0 +1,15 @@
+; RUN: llc < %s -march=x86 | FileCheck %s
+
+ at 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
+}
+
-------------- next part --------------
A non-text attachment was scrubbed...
Name: D344.1.patch
Type: text/x-patch
Size: 1542 bytes
Desc: not available
URL: <http://lists.llvm.org/pipermail/llvm-commits/attachments/20130129/43ba1141/attachment.bin>