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[AArch64] - Fold and and cmp into tst (#110347)
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Fixes llvm/llvm-project#102703.

https://godbolt.org/z/nfj8xsb1Y

The following pattern:

```
%2 = and i32 %0, 254
%3 = icmp eq i32 %2, 0
```
is optimised by instcombine into:

```%3 = icmp ult i32 %0, 2```

However, post instcombine leads to worse aarch64 than the unoptimised version.

Pre instcombine:
```
        tst     w0, #0xfe
        cset    w0, eq
        ret
```
Post instcombine:
```
        and     w8, w0, #0xff
        cmp     w8, #2
        cset    w0, lo
        ret
```


In the unoptimised version, SelectionDAG converts `SETCC (AND X 254) 0 EQ` into `CSEL 0 1 1 (ANDS X 254)`, which gets emitted as a `tst`.

In the optimised version, SelectionDAG converts `SETCC (AND X 255) 2 ULT` into `CSEL 0 1 2 (SUBS (AND X 255) 2)`, which gets emitted as an `and`/`cmp`.

This PR adds an optimisation to `AArch64ISelLowering`, converting `SETCC (AND X Y) Z ULT` into `SETCC (AND X (Y & ~(Z - 1))) 0 EQ` when `Z` is a power of two. This makes SelectionDAG/Codegen produce the same optimised code for both examples.
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@jf-botto
jf-botto authored Oct 3, 2024
1 parent b0c9f02 commit a4516da
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26 changes: 26 additions & 0 deletions llvm/lib/Target/AArch64/AArch64ISelLowering.cpp
View file
Original file line number Diff line number Diff line change
Expand Up @@ -4301,6 +4301,29 @@ static SDValue LowerPREFETCH(SDValue Op, SelectionDAG &DAG) {
Op.getOperand(1));
}

// Converts SETCC (AND X Y) Z ULT -> SETCC (AND X (Y & ~(Z - 1)) 0 EQ when Y is
// a power of 2. This is then lowered to ANDS X (Y & ~(Z - 1)) instead of SUBS
// (AND X Y) Z which produces a better opt with EmitComparison
static void simplifySetCCIntoEq(ISD::CondCode &CC, SDValue &LHS, SDValue &RHS,
SelectionDAG &DAG, const SDLoc dl) {
if (CC == ISD::SETULT && LHS.getOpcode() == ISD::AND && LHS->hasOneUse()) {
ConstantSDNode *LHSConstOp = dyn_cast<ConstantSDNode>(LHS.getOperand(1));
ConstantSDNode *RHSConst = dyn_cast<ConstantSDNode>(RHS);
if (LHSConstOp && RHSConst) {
uint64_t LHSConstValue = LHSConstOp->getZExtValue();
uint64_t RHSConstant = RHSConst->getZExtValue();
if (isPowerOf2_64(RHSConstant)) {
uint64_t NewMaskValue = LHSConstValue & ~(RHSConstant - 1);
LHS =
DAG.getNode(ISD::AND, dl, LHS.getValueType(), LHS.getOperand(0),
DAG.getConstant(NewMaskValue, dl, LHS.getValueType()));
RHS = DAG.getConstant(0, dl, RHS.getValueType());
CC = ISD::SETEQ;
}
}
}
}

SDValue AArch64TargetLowering::LowerFP_EXTEND(SDValue Op,
SelectionDAG &DAG) const {
EVT VT = Op.getValueType();
Expand Down Expand Up @@ -10596,6 +10619,9 @@ SDValue AArch64TargetLowering::LowerSETCC(SDValue Op, SelectionDAG &DAG) const {
}

if (LHS.getValueType().isInteger()) {

simplifySetCCIntoEq(CC, LHS, RHS, DAG, dl);

SDValue CCVal;
SDValue Cmp = getAArch64Cmp(
LHS, RHS, ISD::getSetCCInverse(CC, LHS.getValueType()), CCVal, DAG, dl);
Expand Down
216 changes: 216 additions & 0 deletions llvm/test/CodeGen/AArch64/icmp-ult-eq-fold.ll
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Original file line number Diff line number Diff line change
@@ -0,0 +1,216 @@
; NOTE: Assertions have been autogenerated by utils/update_llc_test_checks.py UTC_ARGS: --version 5
; RUN: llc < %s -mtriple=aarch64 | FileCheck %s


define i1 @lt8_u8(i8 %0) {
; CHECK-LABEL: lt8_u8:
; CHECK: // %bb.0:
; CHECK-NEXT: tst w0, #0xf8
; CHECK-NEXT: cset w0, eq
; CHECK-NEXT: ret
%2 = icmp ult i8 %0, 8
ret i1 %2
}

define i1 @lt32_u8(i8 %0) {
; CHECK-LABEL: lt32_u8:
; CHECK: // %bb.0:
; CHECK-NEXT: tst w0, #0xe0
; CHECK-NEXT: cset w0, eq
; CHECK-NEXT: ret
%2 = icmp ult i8 %0, 32
ret i1 %2
}

define i1 @lt64_u8(i8 %0) {
; CHECK-LABEL: lt64_u8:
; CHECK: // %bb.0:
; CHECK-NEXT: tst w0, #0xc0
; CHECK-NEXT: cset w0, eq
; CHECK-NEXT: ret
%2 = icmp ult i8 %0, 64
ret i1 %2
}

define i1 @lt8_u32(i32 %0) {
; CHECK-LABEL: lt8_u32:
; CHECK: // %bb.0:
; CHECK-NEXT: cmp w0, #8
; CHECK-NEXT: cset w0, lo
; CHECK-NEXT: ret
%2 = icmp ult i32 %0, 8
ret i1 %2
}

define i1 @lt32_u32(i32 %0) {
; CHECK-LABEL: lt32_u32:
; CHECK: // %bb.0:
; CHECK-NEXT: cmp w0, #32
; CHECK-NEXT: cset w0, lo
; CHECK-NEXT: ret
%2 = icmp ult i32 %0, 32
ret i1 %2
}

define i1 @lt64_u32(i32 %0) {
; CHECK-LABEL: lt64_u32:
; CHECK: // %bb.0:
; CHECK-NEXT: cmp w0, #64
; CHECK-NEXT: cset w0, lo
; CHECK-NEXT: ret
%2 = icmp ult i32 %0, 64
ret i1 %2
}

define i1 @lt8_u64(i64 %0) {
; CHECK-LABEL: lt8_u64:
; CHECK: // %bb.0:
; CHECK-NEXT: cmp x0, #8
; CHECK-NEXT: cset w0, lo
; CHECK-NEXT: ret
%2 = icmp ult i64 %0, 8
ret i1 %2
}

define i1 @lt32_u64(i64 %0) {
; CHECK-LABEL: lt32_u64:
; CHECK: // %bb.0:
; CHECK-NEXT: cmp x0, #32
; CHECK-NEXT: cset w0, lo
; CHECK-NEXT: ret
%2 = icmp ult i64 %0, 32
ret i1 %2
}

define i1 @lt64_u64(i64 %0) {
; CHECK-LABEL: lt64_u64:
; CHECK: // %bb.0:
; CHECK-NEXT: cmp x0, #64
; CHECK-NEXT: cset w0, lo
; CHECK-NEXT: ret
%2 = icmp ult i64 %0, 64
ret i1 %2
}

define i1 @lt8_u16_and_5(i8 %0) {
; CHECK-LABEL: lt8_u16_and_5:
; CHECK: // %bb.0:
; CHECK-NEXT: mov w8, wzr
; CHECK-NEXT: cmp w8, #0
; CHECK-NEXT: cset w0, eq
; CHECK-NEXT: ret
%2 = and i8 %0, 5
%3 = icmp ult i8 %2, 16
ret i1 %3
}

define i1 @lt8_u16_and_19(i8 %0) {
; CHECK-LABEL: lt8_u16_and_19:
; CHECK: // %bb.0:
; CHECK-NEXT: tst w0, #0x10
; CHECK-NEXT: cset w0, eq
; CHECK-NEXT: ret
%2 = and i8 %0, 19
%3 = icmp ult i8 %2, 16
ret i1 %3
}

define i1 @lt32_u16_and_7(i32 %0) {
; CHECK-LABEL: lt32_u16_and_7:
; CHECK: // %bb.0:
; CHECK-NEXT: mov w8, wzr
; CHECK-NEXT: cmp w8, #0
; CHECK-NEXT: cset w0, eq
; CHECK-NEXT: ret
%2 = and i32 %0, 7
%3 = icmp ult i32 %2, 16
ret i1 %3
}

define i1 @lt32_u16_and_21(i32 %0) {
; CHECK-LABEL: lt32_u16_and_21:
; CHECK: // %bb.0:
; CHECK-NEXT: tst w0, #0x10
; CHECK-NEXT: cset w0, eq
; CHECK-NEXT: ret
%2 = and i32 %0, 21
%3 = icmp ult i32 %2, 16
ret i1 %3
}

define i1 @lt64_u16_and_9(i64 %0) {
; CHECK-LABEL: lt64_u16_and_9:
; CHECK: // %bb.0:
; CHECK-NEXT: mov x8, xzr
; CHECK-NEXT: cmp x8, #0
; CHECK-NEXT: cset w0, eq
; CHECK-NEXT: ret
%2 = and i64 %0, 9
%3 = icmp ult i64 %2, 16
ret i1 %3
}

define i1 @lt64_u16_and_23(i64 %0) {
; CHECK-LABEL: lt64_u16_and_23:
; CHECK: // %bb.0:
; CHECK-NEXT: tst x0, #0x10
; CHECK-NEXT: cset w0, eq
; CHECK-NEXT: ret
%2 = and i64 %0, 23
%3 = icmp ult i64 %2, 16
ret i1 %3
}

; negative test
define i1 @lt3_u8(i8 %0) {
; CHECK-LABEL: lt3_u8:
; CHECK: // %bb.0:
; CHECK-NEXT: and w8, w0, #0xff
; CHECK-NEXT: cmp w8, #3
; CHECK-NEXT: cset w0, lo
; CHECK-NEXT: ret
%2 = icmp ult i8 %0, 3
ret i1 %2
}

; negative test
define i1 @lt3_u32(i32 %0) {
; CHECK-LABEL: lt3_u32:
; CHECK: // %bb.0:
; CHECK-NEXT: cmp w0, #3
; CHECK-NEXT: cset w0, lo
; CHECK-NEXT: ret
%2 = icmp ult i32 %0, 3
ret i1 %2
}

; negative test
define i1 @lt3_u64(i64 %0) {
; CHECK-LABEL: lt3_u64:
; CHECK: // %bb.0:
; CHECK-NEXT: cmp x0, #3
; CHECK-NEXT: cset w0, lo
; CHECK-NEXT: ret
%2 = icmp ult i64 %0, 3
ret i1 %2
}

; negative test
define i32 @lt32_u16_multiple_use(i32 %0) {
; CHECK-LABEL: lt32_u16_multiple_use:
; CHECK: // %bb.0:
; CHECK-NEXT: mov w8, #21 // =0x15
; CHECK-NEXT: mov w9, #10 // =0xa
; CHECK-NEXT: and w8, w0, w8
; CHECK-NEXT: cmp w8, #16
; CHECK-NEXT: orr w8, w8, w9
; CHECK-NEXT: cset w10, lo
; CHECK-NEXT: mul w0, w8, w10
; CHECK-NEXT: ret
%2 = and i32 %0, 21
%3 = icmp ult i32 %2, 16
%4 = add i32 %2, 10
%5 = zext i1 %3 to i32
%6 = mul i32 %4, %5
ret i32 %6
}
25 changes: 10 additions & 15 deletions llvm/test/CodeGen/AArch64/signed-truncation-check.ll
View file
Original file line number Diff line number Diff line change
Expand Up @@ -287,9 +287,8 @@ define i1 @add_ultcmp_bad_i16_i8_add(i16 %x, i16 %y) nounwind {
; CHECK-LABEL: add_ultcmp_bad_i16_i8_add:
; CHECK: // %bb.0:
; CHECK-NEXT: add w8, w0, w1
; CHECK-NEXT: and w8, w8, #0xffff
; CHECK-NEXT: cmp w8, #256
; CHECK-NEXT: cset w0, lo
; CHECK-NEXT: tst w8, #0xff00
; CHECK-NEXT: cset w0, eq
; CHECK-NEXT: ret
%tmp0 = add i16 %x, %y
%tmp1 = icmp ult i16 %tmp0, 256 ; 1U << 8
Expand Down Expand Up @@ -328,9 +327,8 @@ define i1 @add_ultcmp_bad_i16_i8_c0notpoweroftwo(i16 %x) nounwind {
; CHECK-LABEL: add_ultcmp_bad_i16_i8_c0notpoweroftwo:
; CHECK: // %bb.0:
; CHECK-NEXT: add w8, w0, #192
; CHECK-NEXT: and w8, w8, #0xffff
; CHECK-NEXT: cmp w8, #256
; CHECK-NEXT: cset w0, lo
; CHECK-NEXT: tst w8, #0xff00
; CHECK-NEXT: cset w0, eq
; CHECK-NEXT: ret
%tmp0 = add i16 %x, 192 ; (1U << (8-1)) + (1U << (8-1-1))
%tmp1 = icmp ult i16 %tmp0, 256 ; 1U << 8
Expand All @@ -356,9 +354,8 @@ define i1 @add_ultcmp_bad_i16_i8_magic(i16 %x) nounwind {
; CHECK-LABEL: add_ultcmp_bad_i16_i8_magic:
; CHECK: // %bb.0:
; CHECK-NEXT: add w8, w0, #64
; CHECK-NEXT: and w8, w8, #0xffff
; CHECK-NEXT: cmp w8, #256
; CHECK-NEXT: cset w0, lo
; CHECK-NEXT: tst w8, #0xff00
; CHECK-NEXT: cset w0, eq
; CHECK-NEXT: ret
%tmp0 = add i16 %x, 64 ; 1U << (8-1-1)
%tmp1 = icmp ult i16 %tmp0, 256 ; 1U << 8
Expand All @@ -370,9 +367,8 @@ define i1 @add_ultcmp_bad_i16_i4(i16 %x) nounwind {
; CHECK-LABEL: add_ultcmp_bad_i16_i4:
; CHECK: // %bb.0:
; CHECK-NEXT: add w8, w0, #8
; CHECK-NEXT: and w8, w8, #0xffff
; CHECK-NEXT: cmp w8, #16
; CHECK-NEXT: cset w0, lo
; CHECK-NEXT: tst w8, #0xfff0
; CHECK-NEXT: cset w0, eq
; CHECK-NEXT: ret
%tmp0 = add i16 %x, 8 ; 1U << (4-1)
%tmp1 = icmp ult i16 %tmp0, 16 ; 1U << 4
Expand All @@ -384,9 +380,8 @@ define i1 @add_ultcmp_bad_i24_i8(i24 %x) nounwind {
; CHECK-LABEL: add_ultcmp_bad_i24_i8:
; CHECK: // %bb.0:
; CHECK-NEXT: add w8, w0, #128
; CHECK-NEXT: and w8, w8, #0xffffff
; CHECK-NEXT: cmp w8, #256
; CHECK-NEXT: cset w0, lo
; CHECK-NEXT: tst w8, #0xffff00
; CHECK-NEXT: cset w0, eq
; CHECK-NEXT: ret
%tmp0 = add i24 %x, 128 ; 1U << (8-1)
%tmp1 = icmp ult i24 %tmp0, 256 ; 1U << 8
Expand Down

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