[PowerPC] Fix infinite loop in peephole CR optimization (PR49509)

[nikic]

Summary

If we encounter a degenerate select node where both operands are the same, then we can continue negating the condition while swapping operands, resulting in an infinite loop. Avoid this by bailout out if both operands are the same.

Fixes https://bugs.llvm.org/show_bug.cgi?id=49509, encountered in stage2 rustc build.

[nemanjai]

LGTM. Thanks for fixing this.

llvm/lib/Target/PowerPC/PPCISelDAGToDAG.cpp

for (SDNode::use_iterator UI = N->use_begin(), UE = N->use_end();
        UI != UE; ++UI) {
     SDNode *User = *UI;
     if (!User->isMachineOpcode())
       return false;
     if (User->getMachineOpcode() != PPC::SELECT_I4 &&
         User->getMachineOpcode() != PPC::SELECT_I8)
       return false;
 
+    SDNode *Op1 = User->getOperand(1).getNode();
     SDNode *Op2 = User->getOperand(2).getNode();
+    // If we have a degenerate select with two equal operands, swapping will
+    // not do anything, and we may run into an infinite loop.
+    if (Op1 == Op2)
+      return false;
+
     if (!Op2->isMachineOpcode())
       return false;
 
     if (Op2->getMachineOpcode() != PPC::LI &&
         Op2->getMachineOpcode() != PPC::LI8)
       return false;
 
     ConstantSDNode *C = dyn_cast<ConstantSDNode>(Op2->getOperand(0));

llvm/test/CodeGen/PowerPC/pr49509.ll

+; NOTE: Assertions have been autogenerated by utils/update_llc_test_checks.py
+; RUN: llc -mtriple=powerpc-unknown-linux-gnu < %s | FileCheck %s
+
+target datalayout = "E-m:e-p:32:32-i64:64-n32"
+
+define void @test() {
+; CHECK-LABEL: test:
+; CHECK:       # %bb.0: # %bb
+; CHECK-NEXT:    bc 12, 20, .LBB0_2
+; CHECK-NEXT:  # %bb.1: # %bb2
+; CHECK-NEXT:    li 3, 0
+; CHECK-NEXT:    stw 3, 0(3)
+; CHECK-NEXT:    lis 3, 256
+; CHECK-NEXT:    stw 3, 0(3)
+; CHECK-NEXT:    blr
+; CHECK-NEXT:  .LBB0_2: # %bb1
+; CHECK-NEXT:    bclr 4, 20, 0
+; CHECK-NEXT:  # %bb.3: # %bb66
+; CHECK-NEXT:    lwz 4, 12(0)
+; CHECK-NEXT:    lwz 5, 8(0)
+; CHECK-NEXT:    lwz 6, 0(0)
+; CHECK-NEXT:    lwz 7, 4(0)
+; CHECK-NEXT:    lbz 3, 0(3)
+; CHECK-NEXT:    and 5, 5, 6
+; CHECK-NEXT:    and 4, 4, 7
+; CHECK-NEXT:    and 4, 4, 5
+; CHECK-NEXT:    cmpwi 3, 0
+; CHECK-NEXT:    lis 3, 256
+; CHECK-NEXT:    lis 7, 512
+; CHECK-NEXT:    bc 12, 2, .LBB0_4
+; CHECK-NEXT:    b .LBB0_5
+; CHECK-NEXT:  .LBB0_4: # %bb66
+; CHECK-NEXT:    li 3, 0
+; CHECK-NEXT:  .LBB0_5: # %bb66
+; CHECK-NEXT:    cmpwi 1, 4, -1
+; CHECK-NEXT:    cmpwi 5, 4, -1
+; CHECK-NEXT:    li 6, 0
+; CHECK-NEXT:    bc 12, 6, .LBB0_6
+; CHECK-NEXT:    b .LBB0_7
+; CHECK-NEXT:  .LBB0_6: # %bb66
+; CHECK-NEXT:    addi 3, 7, 0
+; CHECK-NEXT:  .LBB0_7: # %bb66
+; CHECK-NEXT:    cror 20, 22, 2
+; CHECK-NEXT:    stw 3, 0(3)
+; CHECK-NEXT:    bc 12, 20, .LBB0_9
+; CHECK-NEXT:  # %bb.8: # %bb66
+; CHECK-NEXT:    ori 3, 6, 0
+; CHECK-NEXT:    b .LBB0_10
+; CHECK-NEXT:  .LBB0_9: # %bb66
+; CHECK-NEXT:    li 3, 0
+; CHECK-NEXT:  .LBB0_10: # %bb66
+; CHECK-NEXT:    stw 3, 0(3)
+; CHECK-NEXT:    blr
+bb:
+  br i1 undef, label %bb2, label %bb1
+
+bb2:                                              ; preds = %bb
+  %i = select i1 undef, i64 0, i64 72057594037927936
+  store i64 %i, i64* undef, align 8
+  ret void
+
+bb1:                                              ; preds = %bb
+  %i50 = load i8, i8* undef, align 8
+  %i52 = load i128, i128* null, align 8
+  %i62 = icmp eq i8 %i50, 0
+  br i1 undef, label %bb66, label %bb64
+
+bb64:                                             ; preds = %bb63
+  ret void
+
+bb66:                                             ; preds = %bb63
+  %i67 = lshr i128 -1, 0
+  %i68 = xor i128 %i52, -1
+  %i69 = add i128 0, %i68
+  %i70 = and i128 %i67, %i69
+  %i71 = icmp eq i128 %i70, 0
+  %i74 = select i1 %i62, i64 0, i64 72057594037927936
+  %i75 = select i1 %i71, i64 144115188075855872, i64 %i74
+  store i64 %i75, i64* undef, align 8
+  ret void
+}