[SystemZ] Fix incorrect use of RISBG for a zero-extended right shift

We would wrongly transform the testcase into the equivalent of an AND with 1.
The problem was that, when testing whether the shifted-in bits of the right
shift were significant, we used the width of the final zero-extended result
rather than the width of the shifted value.

llvm-svn: 195731

2 files changed, 22 insertions, 19 deletions

diff --git a/llvm/lib/Target/SystemZ/SystemZISelDAGToDAG.cpp b/llvm/lib/Target/SystemZ/SystemZISelDAGToDAG.cpp
index 7febed25cb8..f4a27733ce0 100644
--- a/llvm/lib/Target/SystemZ/SystemZISelDAGToDAG.cpp
+++ b/llvm/lib/Target/SystemZ/SystemZISelDAGToDAG.cpp
@@ -688,23 +688,12 @@ bool SystemZDAGToDAGISel::refineRxSBGMask(RxSBGOperands &RxSBG,
   return false;
 }
 
-// RxSBG.Input is a shift of Count bits in the direction given by IsLeft.
-// Return true if the result depends on the signs or zeros that are
-// shifted in.
-static bool shiftedInBitsMatter(RxSBGOperands &RxSBG, uint64_t Count,
-                                bool IsLeft) {
-  // Work out which bits of the shift result are zeros or sign copies.
-  uint64_t ShiftedIn = allOnes(Count);
-  if (!IsLeft)
-    ShiftedIn <<= RxSBG.BitSize - Count;
-
-  // Rotate that mask in the same way as RxSBG.Input is rotated.
+// Return true if any bits of (RxSBG.Input & Mask) are significant.
+static bool maskMatters(RxSBGOperands &RxSBG, uint64_t Mask) {
+  // Rotate the mask in the same way as RxSBG.Input is rotated.
   if (RxSBG.Rotate != 0)
-    ShiftedIn = ((ShiftedIn << RxSBG.Rotate) |
-                 (ShiftedIn >> (64 - RxSBG.Rotate)));
-
-  // Fail if any of the zero or sign bits are used.
-  return (ShiftedIn & RxSBG.Mask) != 0;
+    Mask = ((Mask << RxSBG.Rotate) | (Mask >> (64 - RxSBG.Rotate)));
+  return (Mask & RxSBG.Mask) != 0;
 }
 
 bool SystemZDAGToDAGISel::expandRxSBG(RxSBGOperands &RxSBG) const {
@@ -781,7 +770,7 @@ bool SystemZDAGToDAGISel::expandRxSBG(RxSBGOperands &RxSBG) const {
     // Check that the extension bits are don't-care (i.e. are masked out
     // by the final mask).
     unsigned InnerBitSize = N.getOperand(0).getValueType().getSizeInBits();
-    if (shiftedInBitsMatter(RxSBG, RxSBG.BitSize - InnerBitSize, false))
+    if (maskMatters(RxSBG, allOnes(RxSBG.BitSize) - allOnes(InnerBitSize)))
       return false;
 
     RxSBG.Input = N.getOperand(0);
@@ -802,7 +791,7 @@ bool SystemZDAGToDAGISel::expandRxSBG(RxSBGOperands &RxSBG) const {
     if (RxSBG.Opcode == SystemZ::RNSBG) {
       // Treat (shl X, count) as (rotl X, size-count) as long as the bottom
       // count bits from RxSBG.Input are ignored.
-      if (shiftedInBitsMatter(RxSBG, Count, true))
+      if (maskMatters(RxSBG, allOnes(Count)))
         return false;
     } else {
       // Treat (shl X, count) as (and (rotl X, count), ~0<<count).
@@ -830,7 +819,7 @@ bool SystemZDAGToDAGISel::expandRxSBG(RxSBGOperands &RxSBG) const {
     if (RxSBG.Opcode == SystemZ::RNSBG || Opcode == ISD::SRA) {
       // Treat (srl|sra X, count) as (rotl X, size-count) as long as the top
       // count bits from RxSBG.Input are ignored.
-      if (shiftedInBitsMatter(RxSBG, Count, false))
+      if (maskMatters(RxSBG, allOnes(Count) << (BitSize - Count)))
         return false;
     } else {
       // Treat (srl X, count), mask) as (and (rotl X, size-count), ~0>>count),
diff --git a/llvm/test/CodeGen/SystemZ/risbg-01.ll b/llvm/test/CodeGen/SystemZ/risbg-01.ll
index 8a5d4874f68..a4d11fdae5b 100644
--- a/llvm/test/CodeGen/SystemZ/risbg-01.ll
+++ b/llvm/test/CodeGen/SystemZ/risbg-01.ll
@@ -456,3 +456,17 @@ define i64 @f40(i64 %foo, i64 *%dest) {
   %and = and i64 %shl, 2147483647
   ret i64 %and
 }
+
+; In this case the sign extension is converted to a pair of 32-bit shifts,
+; which is then extended to 64 bits.  We previously used the wrong bit size
+; when testing whether the shifted-in bits of the shift right were significant.
+define i64 @f41(i1 %x) {
+; CHECK-LABEL: f41:
+; CHECK: sll %r2, 31
+; CHECK: sra %r2, 31
+; CHECK: llgcr %r2, %r2
+; CHECK: br %r14
+  %ext = sext i1 %x to i8
+  %ext2 = zext i8 %ext to i64
+  ret i64 %ext2
+}