Fix PR4254.

The DAGCombiner created a negative shiftamount, stored in an
unsigned variable. Later the optimizer eliminated the shift entirely as being
undefined.
Example: (srl (shl X, 56) 48). ShiftAmt is 4294967288.
Fix it by checking that the shiftamount is positive, and storing in a signed
variable.

llvm-svn: 72331

2 files changed, 16 insertions, 2 deletions

diff --git a/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp b/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
index 6e0cc9f60a2..a866cb5629e 100644
--- a/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
+++ b/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
@@ -2546,13 +2546,13 @@ SDValue DAGCombiner::visitSRA(SDNode *N) {
       MVT TruncVT =
         MVT::getIntegerVT(VTValSize - N1C->getZExtValue());
       // Determine the residual right-shift amount.
-      unsigned ShiftAmt = N1C->getZExtValue() - N01C->getZExtValue();
+      signed ShiftAmt = N1C->getZExtValue() - N01C->getZExtValue();
 
       // If the shift is not a no-op (in which case this should be just a sign
       // extend already), the truncated to type is legal, sign_extend is legal
       // on that type, and the the truncate to that type is both legal and free,
       // perform the transform.
-      if (ShiftAmt &&
+      if ((ShiftAmt > 0) &&
           TLI.isOperationLegalOrCustom(ISD::SIGN_EXTEND, TruncVT) &&
           TLI.isOperationLegalOrCustom(ISD::TRUNCATE, VT) &&
           TLI.isTruncateFree(VT, TruncVT)) {
diff --git a/llvm/test/CodeGen/X86/2009-05-23-dagcombine-shifts.ll b/llvm/test/CodeGen/X86/2009-05-23-dagcombine-shifts.ll
new file mode 100644
index 00000000000..6f2bef4fca1
--- /dev/null
+++ b/llvm/test/CodeGen/X86/2009-05-23-dagcombine-shifts.ll
@@ -0,0 +1,14 @@
+; RUN: llvm-as < %s | llc | grep -E {sar|shl|mov|or} | count 4
+; Check that the shr(shl X, 56), 48) is not mistakenly turned into
+; a shr (X, -8) that gets subsequently "optimized away" as undef
+; PR4254
+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-f80:128:128"
+target triple = "x86_64-unknown-linux-gnu"
+
+define i64 @foo(i64 %b) nounwind readnone {
+entry:
+	%shl = shl i64 %b, 56		; <i64> [#uses=1]
+	%shr = ashr i64 %shl, 48		; <i64> [#uses=1]
+	%add5 = or i64 %shr, 1		; <i64> [#uses=1]
+	ret i64 %add5
+}