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May 30, 2024
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[NVPTX] Improve folding to mad with immediate 1 #93628

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[NVPTX] Improve folding to mad with immediate 1
AlexMaclean May 29, 2024
af084ad
address comments
AlexMaclean May 29, 2024
d0c2760
address comments
AlexMaclean May 29, 2024
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96 changes: 90 additions & 6 deletions llvm/lib/Target/NVPTX/NVPTXISelLowering.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -5614,17 +5614,101 @@ static SDValue TryMULWIDECombine(SDNode *N,
return DCI.DAG.getNode(Opc, DL, MulType, TruncLHS, TruncRHS);
}

static SDValue matchMADConstOnePattern(SDValue Add) {
if (Add->getOpcode() != ISD::ADD)
return SDValue();

if (const auto *Const0 = dyn_cast<ConstantSDNode>(Add->getOperand(0)))
if (Const0->getZExtValue() == 1)
return Add->getOperand(1);

if (const auto *Const1 = dyn_cast<ConstantSDNode>(Add->getOperand(1)))
if (Const1->getZExtValue() == 1)
return Add->getOperand(0);

return SDValue();
}

static SDValue combineMADConstOne(SDValue X, SDValue Add, EVT VT, SDLoc DL,
TargetLowering::DAGCombinerInfo &DCI) {

if (SDValue Y = matchMADConstOnePattern(Add))
return DCI.DAG.getNode(NVPTXISD::IMAD, DL, VT, X, Y, X);

return SDValue();
}

static SDValue combineMulSelectConstOne(SDValue X, SDValue Select, EVT VT,
SDLoc DL,
TargetLowering::DAGCombinerInfo &DCI) {
if (Select->getOpcode() != ISD::SELECT)
return SDValue();

SDValue Cond = Select->getOperand(0);

unsigned ConstOpNo = 1;
auto *Const = dyn_cast<ConstantSDNode>(Select->getOperand(ConstOpNo));
if (!Const || Const->getZExtValue() != 1) {
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It looks like we could extract the common pattern into a helper function:

bool isConstOne(Operand) {
  const auto *Const = dyn_cast<ConstantSDNode>(Operand);
  return Const && Const->getZExtValue() == 1;
}

and then use it in handful of instances of this pattern throughout the code.

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Nice

ConstOpNo = 2;
Const = dyn_cast<ConstantSDNode>(Select->getOperand(ConstOpNo));
if (!Const || Const->getZExtValue() != 1)
return SDValue();
}

SDValue Y = Select->getOperand((ConstOpNo == 1) ? 2 : 1);

// Do not combine if the resulting sequence is not obviously profitable.
if (!matchMADConstOnePattern(Y))
return SDValue();

SDValue NewMul = DCI.DAG.getNode(ISD::MUL, DL, VT, X, Y);

return DCI.DAG.getNode(ISD::SELECT, DL, VT, Cond,
(ConstOpNo == 1) ? X : NewMul,
(ConstOpNo == 1) ? NewMul : X);
}

static SDValue
PerformMULCombineWithOperands(SDNode *N, SDValue N0, SDValue N1,
TargetLowering::DAGCombinerInfo &DCI) {

EVT VT = N0.getValueType();
if (VT.isVector())
return SDValue();

if (VT != MVT::i16 && VT != MVT::i32 && VT != MVT::i64)
return SDValue();

SDLoc DL(N);

// (mul x, (add y, 1)) -> (mad x, y, x)
if (SDValue Res = combineMADConstOne(N0, N1, VT, DL, DCI))
return Res;
if (SDValue Res = combineMADConstOne(N1, N0, VT, DL, DCI))
return Res;

// (mul x, (select y, 1)) -> (select (mul x, y), x)
if (SDValue Res = combineMulSelectConstOne(N0, N1, VT, DL, DCI))
return Res;
if (SDValue Res = combineMulSelectConstOne(N1, N0, VT, DL, DCI))
return Res;

return SDValue();
}

/// PerformMULCombine - Runs PTX-specific DAG combine patterns on MUL nodes.
static SDValue PerformMULCombine(SDNode *N,
TargetLowering::DAGCombinerInfo &DCI,
CodeGenOptLevel OptLevel) {
if (OptLevel > CodeGenOptLevel::None) {
// Try mul.wide combining at OptLevel > 0
if (SDValue Ret = TryMULWIDECombine(N, DCI))
return Ret;
}
if (OptLevel == CodeGenOptLevel::None)
return SDValue();

return SDValue();
if (SDValue Ret = TryMULWIDECombine(N, DCI))
return Ret;

SDValue N0 = N->getOperand(0);
SDValue N1 = N->getOperand(1);
return PerformMULCombineWithOperands(N, N0, N1, DCI);
}

/// PerformSHLCombine - Runs PTX-specific DAG combine patterns on SHL nodes.
Expand Down
136 changes: 136 additions & 0 deletions llvm/test/CodeGen/NVPTX/combine-mad.ll
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@@ -0,0 +1,136 @@
; NOTE: Assertions have been autogenerated by utils/update_llc_test_checks.py UTC_ARGS: --version 5
; RUN: llc < %s -mtriple=nvptx -mcpu=sm_20 -O1 | FileCheck %s
; RUN: llc < %s -mtriple=nvptx64 -mcpu=sm_20 -O1 | FileCheck %s
; RUN: %if ptxas %{ llc < %s -mtriple=nvptx -mcpu=sm_20 -O1 | %ptxas-verify %}
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This needs to be disabled with newer ptxas.

RUN: %if ptxas && !ptxas-12.0 %{ llc < %s -march=nvptx -mcpu=sm_20 | %ptxas-verify %}

; RUN: %if ptxas %{ llc < %s -mtriple=nvptx64 -mcpu=sm_20 -O1 | %ptxas-verify %}

define i32 @test1(i32 %n, i32 %m) {
;
; CHECK-LABEL: test1(
; CHECK: {
; CHECK-NEXT: .reg .b32 %r<4>;
; CHECK-EMPTY:
; CHECK-NEXT: // %bb.0:
; CHECK-NEXT: ld.param.u32 %r1, [test1_param_0];
; CHECK-NEXT: ld.param.u32 %r2, [test1_param_1];
; CHECK-NEXT: mad.lo.s32 %r3, %r2, %r1, %r2;
; CHECK-NEXT: st.param.b32 [func_retval0+0], %r3;
; CHECK-NEXT: ret;
%add = add i32 %n, 1
%mul = mul i32 %add, %m
ret i32 %mul
}

define i32 @test1_rev(i32 %n, i32 %m) {
;
; CHECK-LABEL: test1_rev(
; CHECK: {
; CHECK-NEXT: .reg .b32 %r<4>;
; CHECK-EMPTY:
; CHECK-NEXT: // %bb.0:
; CHECK-NEXT: ld.param.u32 %r1, [test1_rev_param_0];
; CHECK-NEXT: ld.param.u32 %r2, [test1_rev_param_1];
; CHECK-NEXT: mad.lo.s32 %r3, %r2, %r1, %r2;
; CHECK-NEXT: st.param.b32 [func_retval0+0], %r3;
; CHECK-NEXT: ret;
%add = add i32 %n, 1
%mul = mul i32 %m, %add
ret i32 %mul
}

; Transpose (mul (select)) if it can then be folded to mad
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Does it buy us anything?

mul(m,select(1,n)) will probably have the same performance as select(mul(m,n), m) as the critical path will always have mul and select, just in different order.

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By itself this transform doesn't help much, I agree. However, if m or n are add(x,1) then it enables the other transformation. In the code we're checking for this case and only running the transformation when it would enable further folding. A rare case to be sure, but better to support it than not.

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This kind of optimization is not target-specific and should probably be done somewhere in instcombine. Perhaps move the optimization of mul(m,select(1,n)) there as a separate patch?

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instcombine already canonicalizes in the opposite direction, select(mul(m,n), m) -> mul(m,select(1,n)). I think this is target specific because it is only worth doing to improve mad folding.

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OK.

define i32 @test2(i32 %n, i32 %m, i32 %s) {
;
; CHECK-LABEL: test2(
; CHECK: {
; CHECK-NEXT: .reg .pred %p<2>;
; CHECK-NEXT: .reg .b32 %r<6>;
; CHECK-EMPTY:
; CHECK-NEXT: // %bb.0:
; CHECK-NEXT: ld.param.u32 %r1, [test2_param_0];
; CHECK-NEXT: ld.param.u32 %r2, [test2_param_1];
; CHECK-NEXT: ld.param.u32 %r3, [test2_param_2];
; CHECK-NEXT: setp.lt.s32 %p1, %r3, 1;
; CHECK-NEXT: mad.lo.s32 %r4, %r2, %r1, %r2;
; CHECK-NEXT: selp.b32 %r5, %r2, %r4, %p1;
; CHECK-NEXT: st.param.b32 [func_retval0+0], %r5;
; CHECK-NEXT: ret;
%add = add i32 %n, 1
%cond = icmp slt i32 %s, 1
%sel = select i1 %cond, i32 1, i32 %add
%mul = mul i32 %sel, %m
ret i32 %mul
}

;; Transpose (mul (select)) if it can then be folded to mad
define i32 @test2_rev1(i32 %n, i32 %m, i32 %s) {
;
; CHECK-LABEL: test2_rev1(
; CHECK: {
; CHECK-NEXT: .reg .pred %p<2>;
; CHECK-NEXT: .reg .b32 %r<6>;
; CHECK-EMPTY:
; CHECK-NEXT: // %bb.0:
; CHECK-NEXT: ld.param.u32 %r1, [test2_rev1_param_0];
; CHECK-NEXT: ld.param.u32 %r2, [test2_rev1_param_1];
; CHECK-NEXT: ld.param.u32 %r3, [test2_rev1_param_2];
; CHECK-NEXT: setp.lt.s32 %p1, %r3, 1;
; CHECK-NEXT: mad.lo.s32 %r4, %r2, %r1, %r2;
; CHECK-NEXT: selp.b32 %r5, %r4, %r2, %p1;
; CHECK-NEXT: st.param.b32 [func_retval0+0], %r5;
; CHECK-NEXT: ret;
%add = add i32 %n, 1
%cond = icmp slt i32 %s, 1
%sel = select i1 %cond, i32 %add, i32 1
%mul = mul i32 %sel, %m
ret i32 %mul
}

;; Transpose (mul (select)) if it can then be folded to mad
define i32 @test2_rev2(i32 %n, i32 %m, i32 %s) {
;
; CHECK-LABEL: test2_rev2(
; CHECK: {
; CHECK-NEXT: .reg .pred %p<2>;
; CHECK-NEXT: .reg .b32 %r<6>;
; CHECK-EMPTY:
; CHECK-NEXT: // %bb.0:
; CHECK-NEXT: ld.param.u32 %r1, [test2_rev2_param_0];
; CHECK-NEXT: ld.param.u32 %r2, [test2_rev2_param_1];
; CHECK-NEXT: ld.param.u32 %r3, [test2_rev2_param_2];
; CHECK-NEXT: setp.lt.s32 %p1, %r3, 1;
; CHECK-NEXT: mad.lo.s32 %r4, %r2, %r1, %r2;
; CHECK-NEXT: selp.b32 %r5, %r4, %r2, %p1;
; CHECK-NEXT: st.param.b32 [func_retval0+0], %r5;
; CHECK-NEXT: ret;
%add = add i32 %n, 1
%cond = icmp slt i32 %s, 1
%sel = select i1 %cond, i32 %add, i32 1
%mul = mul i32 %m, %sel
ret i32 %mul
}

;; Leave (mul (select)) intact if it transposing is not profitable
define i32 @test3(i32 %n, i32 %m, i32 %s) {
;
; CHECK-LABEL: test3(
; CHECK: {
; CHECK-NEXT: .reg .pred %p<2>;
; CHECK-NEXT: .reg .b32 %r<7>;
; CHECK-EMPTY:
; CHECK-NEXT: // %bb.0:
; CHECK-NEXT: ld.param.u32 %r1, [test3_param_0];
; CHECK-NEXT: add.s32 %r2, %r1, 3;
; CHECK-NEXT: ld.param.u32 %r3, [test3_param_1];
; CHECK-NEXT: ld.param.u32 %r4, [test3_param_2];
; CHECK-NEXT: setp.lt.s32 %p1, %r4, 1;
; CHECK-NEXT: selp.b32 %r5, 1, %r2, %p1;
; CHECK-NEXT: mul.lo.s32 %r6, %r5, %r3;
; CHECK-NEXT: st.param.b32 [func_retval0+0], %r6;
; CHECK-NEXT: ret;
%add = add i32 %n, 3
%cond = icmp slt i32 %s, 1
%sel = select i1 %cond, i32 1, i32 %add
%mul = mul i32 %sel, %m
ret i32 %mul
}
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