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[InstCombine] Fold X udiv Y to X lshr cttz(Y) if Y is a power of 2 #121386

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Merged
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Jan 11, 2025
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[InstCombine] Fold X udiv Y to X lshr cttz(Y) if Y is a power of 2 #121386

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72b6b01
Add Test
veera-sivarajan Dec 31, 2024
7b4831d
Fold `X udiv Y` to `X lshr cttz(Y)` if Y is a power of 2
veera-sivarajan Jan 9, 2025
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19 changes: 17 additions & 2 deletions llvm/lib/Transforms/InstCombine/InstCombineMulDivRem.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -1598,8 +1598,23 @@ Instruction *InstCombinerImpl::visitUDiv(BinaryOperator &I) {
return Lshr;
}

// Op1 udiv Op2 -> Op1 lshr log2(Op2), if log2() folds away.
if (Value *Res = tryGetLog2(Op1, /*AssumeNonZero=*/true))
auto GetShiftableDenom = [&](Value *Denom) -> Value * {
// Op0 udiv Op1 -> Op0 lshr log2(Op1), if log2() folds away.
if (Value *Log2 = tryGetLog2(Op1, /*AssumeNonZero=*/true))
return Log2;

// Op0 udiv Op1 -> Op0 lshr cttz(Op1), if Op1 is a power of 2.
if (isKnownToBeAPowerOfTwo(Denom, /*OrZero=*/true, /*Depth=*/0, &I))
// This will increase instruction count but it's okay
// since bitwise operations are substantially faster than
// division.
return Builder.CreateBinaryIntrinsic(Intrinsic::cttz, Denom,
Builder.getTrue());

return nullptr;
};

if (auto *Res = GetShiftableDenom(Op1))
return replaceInstUsesWith(
I, Builder.CreateLShr(Op0, Res, I.getName(), I.isExact()));

Expand Down
6 changes: 4 additions & 2 deletions llvm/test/Transforms/IndVarSimplify/rewrite-loop-exit-value.ll
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Expand Up @@ -218,7 +218,8 @@ define i32 @vscale_slt_with_vp_umin(ptr nocapture %A, i32 %n) mustprogress vscal
; CHECK-NEXT: br i1 [[CMP]], label [[FOR_BODY]], label [[FOR_END:%.*]]
; CHECK: for.end:
; CHECK-NEXT: [[TMP0:%.*]] = add nsw i32 [[N]], -1
; CHECK-NEXT: [[TMP1:%.*]] = udiv i32 [[TMP0]], [[VF]]
; CHECK-NEXT: [[TMP5:%.*]] = call range(i32 2, 33) i32 @llvm.cttz.i32(i32 [[VF]], i1 true)
; CHECK-NEXT: [[TMP1:%.*]] = lshr i32 [[TMP0]], [[TMP5]]
; CHECK-NEXT: [[TMP2:%.*]] = mul i32 [[TMP1]], [[VSCALE]]
; CHECK-NEXT: [[TMP3:%.*]] = shl i32 [[TMP2]], 2
; CHECK-NEXT: [[TMP4:%.*]] = sub i32 [[N]], [[TMP3]]
Expand Down Expand Up @@ -270,7 +271,8 @@ define i32 @vscale_slt_with_vp_umin2(ptr nocapture %A, i32 %n) mustprogress vsca
; CHECK-NEXT: br i1 [[CMP]], label [[FOR_BODY]], label [[FOR_END:%.*]]
; CHECK: for.end:
; CHECK-NEXT: [[TMP0:%.*]] = add i32 [[N]], -1
; CHECK-NEXT: [[TMP1:%.*]] = udiv i32 [[TMP0]], [[VF]]
; CHECK-NEXT: [[TMP5:%.*]] = call range(i32 2, 33) i32 @llvm.cttz.i32(i32 [[VF]], i1 true)
; CHECK-NEXT: [[TMP1:%.*]] = lshr i32 [[TMP0]], [[TMP5]]
; CHECK-NEXT: [[TMP2:%.*]] = mul i32 [[TMP1]], [[VSCALE]]
; CHECK-NEXT: [[TMP3:%.*]] = shl i32 [[TMP2]], 2
; CHECK-NEXT: [[TMP4:%.*]] = sub i32 [[N]], [[TMP3]]
Expand Down
113 changes: 109 additions & 4 deletions llvm/test/Transforms/InstCombine/div-shift.ll
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Expand Up @@ -148,7 +148,8 @@ define i8 @udiv_umin_extra_use(i8 %x, i8 %y, i8 %z) {
; CHECK-NEXT: [[Z2:%.*]] = shl nuw i8 1, [[Z:%.*]]
; CHECK-NEXT: [[M:%.*]] = call i8 @llvm.umin.i8(i8 [[Y2]], i8 [[Z2]])
; CHECK-NEXT: call void @use(i8 [[M]])
; CHECK-NEXT: [[D:%.*]] = udiv i8 [[X:%.*]], [[M]]
; CHECK-NEXT: [[TMP1:%.*]] = call range(i8 0, 9) i8 @llvm.cttz.i8(i8 [[M]], i1 true)
; CHECK-NEXT: [[D:%.*]] = lshr i8 [[X:%.*]], [[TMP1]]
; CHECK-NEXT: ret i8 [[D]]
;
%y2 = shl i8 1, %y
Expand All @@ -165,7 +166,8 @@ define i8 @udiv_smin(i8 %x, i8 %y, i8 %z) {
; CHECK-NEXT: [[Y2:%.*]] = shl nuw i8 1, [[Y:%.*]]
; CHECK-NEXT: [[Z2:%.*]] = shl nuw i8 1, [[Z:%.*]]
; CHECK-NEXT: [[M:%.*]] = call i8 @llvm.smin.i8(i8 [[Y2]], i8 [[Z2]])
; CHECK-NEXT: [[D:%.*]] = udiv i8 [[X:%.*]], [[M]]
; CHECK-NEXT: [[TMP1:%.*]] = call range(i8 0, 9) i8 @llvm.cttz.i8(i8 [[M]], i1 true)
; CHECK-NEXT: [[D:%.*]] = lshr i8 [[X:%.*]], [[TMP1]]
; CHECK-NEXT: ret i8 [[D]]
;
%y2 = shl i8 1, %y
Expand All @@ -181,7 +183,8 @@ define i8 @udiv_smax(i8 %x, i8 %y, i8 %z) {
; CHECK-NEXT: [[Y2:%.*]] = shl nuw i8 1, [[Y:%.*]]
; CHECK-NEXT: [[Z2:%.*]] = shl nuw i8 1, [[Z:%.*]]
; CHECK-NEXT: [[M:%.*]] = call i8 @llvm.smax.i8(i8 [[Y2]], i8 [[Z2]])
; CHECK-NEXT: [[D:%.*]] = udiv i8 [[X:%.*]], [[M]]
; CHECK-NEXT: [[TMP1:%.*]] = call range(i8 0, 9) i8 @llvm.cttz.i8(i8 [[M]], i1 true)
; CHECK-NEXT: [[D:%.*]] = lshr i8 [[X:%.*]], [[TMP1]]
; CHECK-NEXT: ret i8 [[D]]
;
%y2 = shl i8 1, %y
Expand Down Expand Up @@ -1006,7 +1009,8 @@ define i8 @udiv_fail_shl_overflow(i8 %x, i8 %y) {
; CHECK-LABEL: @udiv_fail_shl_overflow(
; CHECK-NEXT: [[SHL:%.*]] = shl i8 2, [[Y:%.*]]
; CHECK-NEXT: [[MIN:%.*]] = call i8 @llvm.umax.i8(i8 [[SHL]], i8 1)
; CHECK-NEXT: [[MUL:%.*]] = udiv i8 [[X:%.*]], [[MIN]]
; CHECK-NEXT: [[TMP1:%.*]] = call range(i8 0, 9) i8 @llvm.cttz.i8(i8 [[MIN]], i1 true)
; CHECK-NEXT: [[MUL:%.*]] = lshr i8 [[X:%.*]], [[TMP1]]
; CHECK-NEXT: ret i8 [[MUL]]
;
%shl = shl i8 2, %y
Expand Down Expand Up @@ -1294,3 +1298,104 @@ entry:
%div = sdiv i32 %add, %add2
ret i32 %div
}

define i8 @udiv_if_power_of_two(i8 %x, i8 %y) {
; CHECK-LABEL: @udiv_if_power_of_two(
; CHECK-NEXT: start:
; CHECK-NEXT: [[TMP0:%.*]] = tail call range(i8 0, 9) i8 @llvm.ctpop.i8(i8 [[Y:%.*]])
; CHECK-NEXT: [[TMP1:%.*]] = icmp eq i8 [[TMP0]], 1
; CHECK-NEXT: br i1 [[TMP1]], label [[BB1:%.*]], label [[BB3:%.*]]
; CHECK: bb1:
; CHECK-NEXT: [[TMP2:%.*]] = call range(i8 0, 9) i8 @llvm.cttz.i8(i8 [[Y]], i1 true)
; CHECK-NEXT: [[TMP3:%.*]] = lshr i8 [[X:%.*]], [[TMP2]]
; CHECK-NEXT: br label [[BB3]]
; CHECK: bb3:
; CHECK-NEXT: [[_0_SROA_0_0:%.*]] = phi i8 [ [[TMP3]], [[BB1]] ], [ 0, [[START:%.*]] ]
; CHECK-NEXT: ret i8 [[_0_SROA_0_0]]
;
start:
%ctpop = tail call i8 @llvm.ctpop.i8(i8 %y)
%cmp = icmp eq i8 %ctpop, 1
br i1 %cmp, label %bb1, label %bb3

bb1:
%div = udiv i8 %x, %y
br label %bb3

bb3:
%result = phi i8 [ %div, %bb1 ], [ 0, %start ]
ret i8 %result
}

define i8 @udiv_exact_assume_power_of_two(i8 %x, i8 %y) {
; CHECK-LABEL: @udiv_exact_assume_power_of_two(
; CHECK-NEXT: start:
; CHECK-NEXT: [[TMP0:%.*]] = tail call range(i8 1, 9) i8 @llvm.ctpop.i8(i8 [[Y:%.*]])
; CHECK-NEXT: [[COND:%.*]] = icmp eq i8 [[TMP0]], 1
; CHECK-NEXT: tail call void @llvm.assume(i1 [[COND]])
; CHECK-NEXT: [[TMP1:%.*]] = call range(i8 0, 9) i8 @llvm.cttz.i8(i8 [[Y]], i1 true)
; CHECK-NEXT: [[_0:%.*]] = lshr exact i8 [[X:%.*]], [[TMP1]]
; CHECK-NEXT: ret i8 [[_0]]
;
start:
%ctpop = tail call i8 @llvm.ctpop.i8(i8 %y)
%cond = icmp eq i8 %ctpop, 1
tail call void @llvm.assume(i1 %cond)
%div = udiv exact i8 %x, %y
ret i8 %div
}

define i7 @udiv_assume_power_of_two_illegal_type(i7 %x, i7 %y) {
; CHECK-LABEL: @udiv_assume_power_of_two_illegal_type(
; CHECK-NEXT: start:
; CHECK-NEXT: [[TMP0:%.*]] = tail call range(i7 1, 8) i7 @llvm.ctpop.i7(i7 [[Y:%.*]])
; CHECK-NEXT: [[COND:%.*]] = icmp eq i7 [[TMP0]], 1
; CHECK-NEXT: tail call void @llvm.assume(i1 [[COND]])
; CHECK-NEXT: [[TMP1:%.*]] = call range(i7 0, 8) i7 @llvm.cttz.i7(i7 [[Y]], i1 true)
; CHECK-NEXT: [[_0:%.*]] = lshr i7 [[X:%.*]], [[TMP1]]
; CHECK-NEXT: ret i7 [[_0]]
;
start:
%ctpop = tail call i7 @llvm.ctpop.i7(i7 %y)
%cond = icmp eq i7 %ctpop, 1
tail call void @llvm.assume(i1 %cond)
%div = udiv i7 %x, %y
ret i7 %div
}

define i8 @udiv_assume_power_of_two_multiuse(i8 %x, i8 %y) {
; CHECK-LABEL: @udiv_assume_power_of_two_multiuse(
; CHECK-NEXT: start:
; CHECK-NEXT: [[TMP0:%.*]] = tail call range(i8 1, 9) i8 @llvm.ctpop.i8(i8 [[Y:%.*]])
; CHECK-NEXT: [[COND:%.*]] = icmp eq i8 [[TMP0]], 1
; CHECK-NEXT: tail call void @llvm.assume(i1 [[COND]])
; CHECK-NEXT: [[TMP1:%.*]] = call range(i8 0, 9) i8 @llvm.cttz.i8(i8 [[Y]], i1 true)
; CHECK-NEXT: [[_0:%.*]] = lshr i8 [[X:%.*]], [[TMP1]]
; CHECK-NEXT: call void @use(i8 [[_0]])
; CHECK-NEXT: ret i8 [[_0]]
;
start:
%ctpop = tail call i8 @llvm.ctpop.i8(i8 %y)
%cond = icmp eq i8 %ctpop, 1
tail call void @llvm.assume(i1 %cond)
%div = udiv i8 %x, %y
call void @use(i8 %div)
ret i8 %div
}

define i8 @udiv_power_of_two_negative(i8 %x, i8 %y, i8 %z) {
; CHECK-LABEL: @udiv_power_of_two_negative(
; CHECK-NEXT: start:
; CHECK-NEXT: [[CTPOP:%.*]] = tail call range(i8 0, 9) i8 @llvm.ctpop.i8(i8 [[Z:%.*]])
; CHECK-NEXT: [[COND:%.*]] = icmp eq i8 [[CTPOP]], 1
; CHECK-NEXT: tail call void @llvm.assume(i1 [[COND]])
; CHECK-NEXT: [[_0:%.*]] = udiv i8 [[X:%.*]], [[Y:%.*]]
; CHECK-NEXT: ret i8 [[_0]]
;
start:
%ctpop = tail call i8 @llvm.ctpop.i8(i8 %z)
%cond = icmp eq i8 %ctpop, 1
tail call void @llvm.assume(i1 %cond)
%div = udiv i8 %x, %y
ret i8 %div
}
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