[PATCH] D154575: [X86] Prevent infinite loop in SelectionDAG when lowering negations

[Maurice Heumann via Phabricator via llvm-commits]

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In certain cases, lowering negations can cause an infinite loop in SelectionDAG on X86.

The following snippet shows that behaviour:
https://godbolt.org/z/5hP45T4hY

What happens is that ADD(XOR(..., -1), 1) is detected as the two's complement and transformed into SUB(0, ...)
However, immediates can not be encoded as the LHS of a SUB on X86.
Therefore it is transformed back into an ADD/XOR pair, which is then again transformed into a SUB and so on.

In that specific case, I still think it is valid to display this as a SUB(0,...) , because it should eventually be lowered as a NEG.
Which seems better than an ADD/XOR pair.

Adding an exception to the X86 specific handling for SUBs with 0 LHS operand fixes this infinite loop.


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D154575

Files:
  llvm/lib/Target/X86/X86ISelLowering.cpp
  llvm/test/CodeGen/X86/select-neg.ll


Index: llvm/test/CodeGen/X86/select-neg.ll
===================================================================
--- /dev/null
+++ llvm/test/CodeGen/X86/select-neg.ll
@@ -0,0 +1,17 @@
+; NOTE: Assertions have been autogenerated by utils/update_llc_test_checks.py
+; RUN: llc < %s -mtriple=i386-unknown-unknown | FileCheck %s
+
+ at value1 = external hidden constant i32
+
+define i32 @function(i32 %arg1) {
+; CHECK-LABEL: function:
+; CHECK:       # %bb.0: # %entry
+; CHECK-NEXT:    movl $value1, %eax
+; CHECK-NEXT:    xorl {{[0-9]+}}(%esp), %eax
+; CHECK-NEXT:    negl %eax
+; CHECK-NEXT:    retl
+entry:
+  %0 = xor i32 %arg1, xor (i32 ptrtoint (i32* @value1 to i32), i32 -1)
+  %.neg = add i32 %0, 1
+  ret i32 %.neg
+}
Index: llvm/lib/Target/X86/X86ISelLowering.cpp
===================================================================
--- llvm/lib/Target/X86/X86ISelLowering.cpp
+++ llvm/lib/Target/X86/X86ISelLowering.cpp
@@ -56587,9 +56587,11 @@
   // X86 can't encode an immediate LHS of a sub. See if we can push the
   // negation into a preceding instruction. If the RHS of the sub is a XOR with
   // one use and a constant, invert the immediate, saving one register.
+  // However, ignore cases where C1 is 0, as those will become a NEG.
   // sub(C1, xor(X, C2)) -> add(xor(X, ~C2), C1+1)
   if (Op1.getOpcode() == ISD::XOR && IsNonOpaqueConstant(Op0) &&
-      IsNonOpaqueConstant(Op1.getOperand(1)) && Op1->hasOneUse()) {
+      !isNullConstant(Op0) && IsNonOpaqueConstant(Op1.getOperand(1)) &&
+      Op1->hasOneUse()) {
     SDLoc DL(N);
     EVT VT = Op0.getValueType();
     SDValue NewXor = DAG.getNode(ISD::XOR, SDLoc(Op1), VT, Op1.getOperand(0),


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