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[M68k] Instruction selection to choose neg x when mul x -1 (Fix issue 48588) 

This patch is trying to fix issue 48588(https://github.com/llvm/llvm-project/issues/48588)

I found the results of Instruction Selection between SelectionDAG and FastISEL for the `%mul = mul i32 %A, 4294967295`:
(seldag-isel) mul --> sub --> SUB32dp
(fast-isel)   mul --> sub --> NEG32d

My patch to fix this issue is by overriding a virtual function M68kDAGToDAGISel::IsProfitableToFold(). Return `false` when it was trying to match with SUB, then it will match with NEG.

Reviewed By: myhsu

Differential Revision: https://reviews.llvm.org/D116886
This commit is contained in:
Douglas Chen 2022-06-03 13:19:03 +08:00 committed by Sheng
parent 271a48e029
commit 78b16ccf2b
2 changed files with 56 additions and 8 deletions
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28
llvm/lib/Target/M68k/M68kISelDAGToDAG.cpp
View File

@ -181,6 +181,7 @@ public:
}
bool runOnMachineFunction(MachineFunction &MF) override;
bool IsProfitableToFold(SDValue N, SDNode *U, SDNode *Root) const override;
private:
/// Keep a pointer to the M68kSubtarget around so that we can
@ -311,6 +312,33 @@ private:
};
} // namespace
bool M68kDAGToDAGISel::IsProfitableToFold(SDValue N, SDNode *U,
SDNode *Root) const {
if (OptLevel == CodeGenOpt::None)
return false;
if (U == Root) {
switch (U->getOpcode()) {
default:
return true;
case M68kISD::SUB:
case ISD::SUB:
// Prefer NEG instruction when zero subtracts a value.
// e.g.
// move.l #0, %d0
// sub.l (4,%sp), %d0
// vs.
// move.l (4,%sp), %d0
// neg.l %d0
if (llvm::isNullConstant(U->getOperand(0)))
return false;
break;
}
}
return true;
}
bool M68kDAGToDAGISel::runOnMachineFunction(MachineFunction &MF) {
Subtarget = &MF.getSubtarget<M68kSubtarget>();
return SelectionDAGISel::runOnMachineFunction(MF);

36
llvm/test/CodeGen/M68k/Arith/imul-neg.ll
View File

@ -1,14 +1,34 @@
; NOTE: Assertions have been autogenerated by utils/update_llc_test_checks.py
; RUN: llc < %s -mtriple=m68k-linux | FileCheck %s
; FIXME: When using SelectionDAGISel, the following cases use
; `sub` rather than the expected `neg`
define i8 @mul255_8(i8 %A) {
; CHECK-LABEL: mul255_8:
; CHECK: .cfi_startproc
; CHECK-NEXT: ; %bb.0:
; CHECK-NEXT: move.b (7,%sp), %d0
; CHECK-NEXT: neg.b %d0
; CHECK-NEXT: rts
%mul = mul i8 %A, 255
ret i8 %mul
}
define i16 @mul65535_16(i16 %A) {
; CHECK-LABEL: mul65535_16:
; CHECK: .cfi_startproc
; CHECK-NEXT: ; %bb.0:
; CHECK-NEXT: move.w (6,%sp), %d0
; CHECK-NEXT: neg.w %d0
; CHECK-NEXT: rts
%mul = mul i16 %A, 65535
ret i16 %mul
}
define i32 @mul4294967295_32(i32 %A) {
; CHECK-LABEL: mul4294967295_32:
; CHECK: .cfi_startproc
; CHECK-NEXT: ; %bb.0:
; CHECK-NEXT: move.l #0, %d0
; CHECK-NEXT: sub.l (4,%sp), %d0
; CHECK-NEXT: move.l (4,%sp), %d0
; CHECK-NEXT: neg.l %d0
; CHECK-NEXT: rts
%mul = mul i32 %A, 4294967295
ret i32 %mul
@ -19,10 +39,10 @@ define i64 @mul18446744073709551615_64(i64 %A) {
; CHECK-LABEL: mul18446744073709551615_64:
; CHECK: .cfi_startproc
; CHECK-NEXT: ; %bb.0:
; CHECK-NEXT: move.l (4,%sp), %d0
; CHECK-NEXT: move.l #0, %d1
; CHECK-NEXT: sub.l (8,%sp), %d1
; CHECK-NEXT: negx.l %d0
; CHECK-NEXT: move.l (4,%sp), %d0
; CHECK-NEXT: move.l (8,%sp), %d1
; CHECK-NEXT: neg.l %d1
; CHECK-NEXT: negx.l %d0
; CHECK-NEXT: rts
%mul = mul i64 %A, 18446744073709551615
ret i64 %mul