[DAGCombine] Avoid CombineZExtLogicopShiftLoad if there is free ZEXT

[Carrot]

Summary

This patch fixes pr39098.

For the attached test case, CombineZExtLogicopShiftLoad can optimize it to

t25: i64 = Constant<1099511627775>
t35: i64 = Constant<0>
  t0: ch = EntryToken
t57: i64,ch = load<(load 4 from `i40* undef`, align 8), zext from i32> t0, undef:i64, undef:i64
    t58: i64 = srl t57, Constant:i8<1>
  t60: i64 = and t58, Constant:i64<524287>
t29: ch = store<(store 5 into `i40* undef`, align 8), trunc to i40> t57:1, t60, undef:i64, undef:i64

But later visitANDLike transforms it to

t25: i64 = Constant<1099511627775>
t35: i64 = Constant<0>
  t0: ch = EntryToken
t57: i64,ch = load<(load 4 from `i40* undef`, align 8), zext from i32> t0, undef:i64, undef:i64
      t61: i32 = truncate t57
    t63: i32 = srl t61, Constant:i8<1>
  t64: i32 = and t63, Constant:i32<524287>
t65: i64 = zero_extend t64
    t58: i64 = srl t57, Constant:i8<1>
  t60: i64 = and t58, Constant:i64<524287>
t29: ch = store<(store 5 into `i40* undef`, align 8), trunc to i40> t57:1, t60, undef:i64, undef:i64

And it triggers CombineZExtLogicopShiftLoad again, causes a dead loop.

Both forms should generate same instructions, CombineZExtLogicopShiftLoad generated IR looks cleaner. But it looks more difficult to prevent visitANDLike to do the transform, so I prevent CombineZExtLogicopShiftLoad to do the transform if the ZExt is free.

[spatel]

This is a large pattern match, and I wonder if we could split it up to make it less likely to conflict with other transforms, but I suppose fixing the hang immediately is more important.

test/CodeGen/X86/pr39098.ll

4

Please make this test independent of undef. We end up making tests like that useless when we improve undef propagation. This should be safer:

define void @test_cancel2(i40* %p1, i40* %p2) {
  %t0 = load i40, i40* %p1, align 8
  %shl414 = shl i40 %t0, 19
  %unsclear415 = and i40 %shl414, 549755813887
  %shr416 = lshr i40 %unsclear415, 20
  %unsclear417 = and i40 %shr416, 549755813887
  store i40 %unsclear417, i40* %p2, align 8
  ret void
}

[spatel]

LGTM

[uabelho]

Thanks!

lib/CodeGen/SelectionDAG/DAGCombiner.cpp

SDValue DAGCombiner::CombineExtLoad(SDNode *N) {
   return SDValue(N, 0); // Return N so it doesn't get rechecked!
 }
 
 // fold (zext (and/or/xor (shl/shr (load x), cst), cst)) ->
 //      (and/or/xor (shl/shr (zextload x), (zext cst)), (zext cst))
 SDValue DAGCombiner::CombineZExtLogicopShiftLoad(SDNode *N) {
   assert(N->getOpcode() == ISD::ZERO_EXTEND);
   EVT VT = N->getValueType(0);
+  EVT OrigVT = N->getOperand(0).getValueType();
+  if (TLI.isZExtFree(OrigVT, VT))
+    return SDValue();
 
   // and/or/xor
   SDValue N0 = N->getOperand(0);
   if (!(N0.getOpcode() == ISD::AND || N0.getOpcode() == ISD::OR ||
         N0.getOpcode() == ISD::XOR) ||
       N0.getOperand(1).getOpcode() != ISD::Constant ||
       (LegalOperations && !TLI.isOperationLegal(N0.getOpcode(), VT)))
     return SDValue();

test/CodeGen/X86/pr39098.ll

+; RUN: llc < %s -mtriple=x86_64-unknown-unknown | FileCheck %s
+
+define dso_local void @test_cancel2(i40* %p1, i40* %p2) {
+; CHECK:       # %entry
+; CHECK-NEXT:  movl    (%rdi), %eax
+; CHECK-NEXT:  shrl    %eax
+; CHECK-NEXT:  andl    $524287, %eax
+; CHECK-NEXT:  movl    %eax, (%rsi)
+; CHECK-NEXT:  movb    $0, 4(%rsi)
+; CHECK-NEXT:  retq
+entry:
+  %0 = load i40, i40* %p1, align 8
+  %shl414 = shl i40 %0, 19
+  %unsclear415 = and i40 %shl414, 549755813887
+  %shr416 = lshr i40 %unsclear415, 20
+  %unsclear417 = and i40 %shr416, 549755813887
+  store i40 %unsclear417, i40* %p2, align 8
+  ret void
+}
+