[ARM] Make PerformSHLSimplify add nodes to the DAG worklist correctly.

Intentionally excluding nodes from the DAGCombine worklist is likely to
lead to weird optimizations and infinite loops, so it's generally a bad
idea.

To avoid the infinite loops, fix DAGCombine to use the
isDesirableToCommuteWithShift target hook before performing the
transforms in question, and implement the target hook in the ARM backend
disable the transforms in question.

Fixes https://bugs.llvm.org/show_bug.cgi?id=38530 . (I don't have a
reduced testcase for that bug. But we should have sufficient test
coverage for PerformSHLSimplify given that we're not playing weird
tricks with the worklist. I can try to bugpoint it if necessary,
though.)

Differential Revision: https://reviews.llvm.org/D50667

llvm-svn: 339734

2 files changed, 5 insertions, 2 deletions

diff --git a/llvm/lib/Target/AArch64/AArch64ISelLowering.cpp b/llvm/lib/Target/AArch64/AArch64ISelLowering.cpp
index ec1143f4e58..cbe48a5dc42 100644
--- a/llvm/lib/Target/AArch64/AArch64ISelLowering.cpp
+++ b/llvm/lib/Target/AArch64/AArch64ISelLowering.cpp
@@ -8473,7 +8473,9 @@ AArch64TargetLowering::getScratchRegisters(CallingConv::ID) const {
 }
 
 bool
-AArch64TargetLowering::isDesirableToCommuteWithShift(const SDNode *N) const {
+AArch64TargetLowering::isDesirableToCommuteWithShift(const SDNode *N,
+                                                     CombineLevel Level) const {
+  N = N->getOperand(0).getNode();
   EVT VT = N->getValueType(0);
     // If N is unsigned bit extraction: ((x >> C) & mask), then do not combine
     // it with shift to let it be lowered to UBFX.
diff --git a/llvm/lib/Target/AArch64/AArch64ISelLowering.h b/llvm/lib/Target/AArch64/AArch64ISelLowering.h
index d783c8a6048..a6d66aeae04 100644
--- a/llvm/lib/Target/AArch64/AArch64ISelLowering.h
+++ b/llvm/lib/Target/AArch64/AArch64ISelLowering.h
@@ -363,7 +363,8 @@ public:
   const MCPhysReg *getScratchRegisters(CallingConv::ID CC) const override;
 
   /// Returns false if N is a bit extraction pattern of (X >> C) & Mask.
-  bool isDesirableToCommuteWithShift(const SDNode *N) const override;
+  bool isDesirableToCommuteWithShift(const SDNode *N,
+                                     CombineLevel Level) const override;
 
   /// Returns true if it is beneficial to convert a load of a constant
   /// to just the constant itself.