[PATCH] D42986: [TargetLowering] try to create -1 constant operand for math ops via demanded bits

[Sanjay Patel via Phabricator via llvm-commits]

spatel created this revision.
spatel added reviewers: craig.topper, efriedma, RKSimon.
Herald added subscribers: javed.absar, mcrosier.

This reverses instcombine's demanded bits' transform which always tries to clear bits in constants.

As noted in PR35792 and shown in the test diffs:
https://bugs.llvm.org/show_bug.cgi?id=35792
...we can do better in codegen by trying to form -1. The x86 sub test shows a missed opportunity. We also don't get the shift case from the bug report, but I thought it'd be best to post this as-is to make sure it looks ok.

I did investigate changing instcombine's behavior, but it would be more work to change canonicalization in IR. Clearing bits / shrinking constants can allow killing instructions, so we'd have to figure out how to not regress those cases.


https://reviews.llvm.org/D42986

Files:
  lib/CodeGen/SelectionDAG/TargetLowering.cpp
  test/CodeGen/AArch64/sub1.ll
  test/CodeGen/X86/bmi.ll
  test/CodeGen/X86/zext-demanded.ll


Index: test/CodeGen/X86/zext-demanded.ll
===================================================================
--- test/CodeGen/X86/zext-demanded.ll
+++ test/CodeGen/X86/zext-demanded.ll
@@ -101,7 +101,7 @@
 define i64 @add_neg_one(i64 %x) {
 ; CHECK-LABEL: add_neg_one:
 ; CHECK:       # %bb.0:
-; CHECK-NEXT:    leal 65535(%rdi), %eax
+; CHECK-NEXT:    leal -1(%rdi), %eax
 ; CHECK-NEXT:    andl %edi, %eax
 ; CHECK-NEXT:    movzwl %ax, %eax
 ; CHECK-NEXT:    retq
@@ -128,8 +128,7 @@
 ; CHECK-LABEL: mul_neg_one:
 ; CHECK:       # %bb.0:
 ; CHECK-NEXT:    movl %edi, %eax
-; CHECK-NEXT:    shll $16, %eax
-; CHECK-NEXT:    subl %edi, %eax
+; CHECK-NEXT:    negl %eax
 ; CHECK-NEXT:    andl %edi, %eax
 ; CHECK-NEXT:    movzwl %ax, %eax
 ; CHECK-NEXT:    retq
Index: test/CodeGen/X86/bmi.ll
===================================================================
--- test/CodeGen/X86/bmi.ll
+++ test/CodeGen/X86/bmi.ll
@@ -813,8 +813,7 @@
 define i64 @blsr_disguised_constant(i64 %x) {
 ; CHECK-LABEL: blsr_disguised_constant:
 ; CHECK:       # %bb.0:
-; CHECK-NEXT:    leal 65535(%rdi), %eax
-; CHECK-NEXT:    andl %edi, %eax
+; CHECK-NEXT:    blsrl %edi, %eax
 ; CHECK-NEXT:    movzwl %ax, %eax
 ; CHECK-NEXT:    retq
   %a1 = and i64 %x, 65535
Index: test/CodeGen/AArch64/sub1.ll
===================================================================
--- test/CodeGen/AArch64/sub1.ll
+++ test/CodeGen/AArch64/sub1.ll
@@ -4,8 +4,7 @@
 define i64 @sub1_disguised_constant(i64 %x) {
 ; CHECK-LABEL: sub1_disguised_constant:
 ; CHECK:       // %bb.0:
-; CHECK-NEXT:    orr w8, wzr, #0xffff
-; CHECK-NEXT:    add w8, w0, w8
+; CHECK-NEXT:    sub w8, w0, #1 // =1
 ; CHECK-NEXT:    and w8, w0, w8
 ; CHECK-NEXT:    and x0, x8, #0xffff
 ; CHECK-NEXT:    ret
Index: lib/CodeGen/SelectionDAG/TargetLowering.cpp
===================================================================
--- lib/CodeGen/SelectionDAG/TargetLowering.cpp
+++ lib/CodeGen/SelectionDAG/TargetLowering.cpp
@@ -1232,6 +1232,21 @@
       }
       return true;
     }
+
+    // If we have a constant operand, we may be able to turn it into -1 if we
+    // do not demand the high bits. This can make the constant smaller to
+    // encode, allow more general folding, or match specialized instruction
+    // patterns (eg, 'blsr' on x86). Don't bother changing 1 to -1 because that
+    // is probably not useful (and could be detrimental).
+    ConstantSDNode *C = isConstOrConstSplat(Op1);
+    APInt HighMask = APInt::getHighBitsSet(NewMask.getBitWidth(), NewMaskLZ);
+    if (C && !C->isAllOnesValue() && !C->isOne() &&
+        (C->getAPIntValue() | HighMask).isAllOnesValue()) {
+      SDValue Neg1 = TLO.DAG.getAllOnesConstant(dl, VT);
+      SDValue NewMath = TLO.DAG.getNode(Op.getOpcode(), dl, VT, Op0, Neg1);
+      return TLO.CombineTo(Op, NewMath);
+    }
+
     LLVM_FALLTHROUGH;
   }
   default:


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