[ARM] Handle all-ones mask explicitly in targetShrinkDemandedConstant.

[efriedma]

Summary

This avoids a potential infinite loop setting and unsetting bits in the mask.

Testcase reduced from a failure on the polly-aosp bot.

[samparker]

LGTM

lib/Target/ARM/ARMISelLowering.cpp

ARMTargetLowering::targetShrinkDemandedConstant(SDValue Op,
 
   // Make sure the RHS really is a constant.
   ConstantSDNode *C = dyn_cast<ConstantSDNode>(Op.getOperand(1));
   if (!C)
     return false;
 
   unsigned Mask = C->getZExtValue();
 
-  // If mask is zero, nothing to do.
-  if (!Mask)
-    return false;
-
   unsigned Demanded = DemandedAPInt.getZExtValue();
   unsigned ShrunkMask = Mask & Demanded;
   unsigned ExpandedMask = Mask | ~Demanded;
 
+  // If the mask is all zeros, let the target-independent code replace the
+  // result with zero.
+  if (ShrunkMask == 0)
+    return false;
+
+  // If the mask is all ones, erase the AND. (Currently, the target-independent
+  // code won't do this, so we have to do it explicitly to avoid an infinite
+  // loop in obscure cases.)
+  if (ExpandedMask == ~0U)
+    return TLO.CombineTo(Op, Op.getOperand(0));
+
   auto IsLegalMask = [ShrunkMask, ExpandedMask](unsigned Mask) -> bool {
     return (ShrunkMask & Mask) == ShrunkMask && (~ExpandedMask & Mask) == 0;
   };
   auto UseMask = [Mask, Op, VT, &TLO](unsigned NewMask) -> bool {
     if (NewMask == Mask)
       return true;
     SDLoc DL(Op);
     SDValue NewC = TLO.DAG.getConstant(NewMask, DL, VT);

test/CodeGen/ARM/demanded-bits-and.ll

+; NOTE: Assertions have been autogenerated by utils/update_llc_test_checks.py
+; RUN: llc -mtriple=arm-eabi < %s | FileCheck %s
+
+; Make sure this doesn't hang, and there are no unnecessary
+; "and" instructions.
+
+define dso_local void @f(i16* %p) {
+; CHECK-LABEL: f:
+; CHECK:       @ %bb.0: @ %entry
+; CHECK-NEXT:  .LBB0_1: @ %bb
+; CHECK-NEXT:    @ =>This Inner Loop Header: Depth=1
+; CHECK-NEXT:    ldrh r1, [r0]
+; CHECK-NEXT:    and r2, r1, #255
+; CHECK-NEXT:    add r3, r2, r1, lsr #8
+; CHECK-NEXT:    add r2, r3, r2
+; CHECK-NEXT:    add r1, r2, r1, lsr #8
+; CHECK-NEXT:    add r1, r1, #2
+; CHECK-NEXT:    lsr r1, r1, #2
+; CHECK-NEXT:    strh r1, [r0]
+; CHECK-NEXT:    b .LBB0_1
+entry:
+  br label %bb
+
+bb:
+  %_p_scalar_ = load i16, i16* %p, align 2
+  %p_and = and i16 %_p_scalar_, 255
+  %p_ = lshr i16 %_p_scalar_, 8
+  %p_add = add nuw nsw i16 %p_, 2
+  %p_add14 = add nuw nsw i16 %p_add, %p_and
+  %p_add18 = add nuw nsw i16 %p_add14, %p_and
+  %p_add19 = add nuw nsw i16 %p_add18, %p_
+  %p_200 = lshr i16 %p_add19, 2
+  store i16 %p_200, i16* %p, align 2
+  br label %bb
+}