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[InstCombine] Funnel shift with negative amount folds to funnel shift in opposite direction (#138334) #138763

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May 7, 2025
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[InstCombine] Funnel shift with negative amount folds to funnel shift in opposite direction (#138334) #138763

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12 changes: 12 additions & 0 deletions llvm/lib/Transforms/InstCombine/InstCombineCalls.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -2306,6 +2306,18 @@ Instruction *InstCombinerImpl::visitCallInst(CallInst &CI) {
return BitOp;
}

// fshl(X, X, Neg(Y)) --> fshr(X, X, Y)
// fshr(X, X, Neg(Y)) --> fshl(X, X, Y)
// if BitWidth is a power-of-2
Value *Y;
if (Op0 == Op1 && isPowerOf2_32(BitWidth) &&
match(II->getArgOperand(2), m_Neg(m_Value(Y)))) {
Module *Mod = II->getModule();
Function *OppositeShift = Intrinsic::getOrInsertDeclaration(
Mod, IID == Intrinsic::fshl ? Intrinsic::fshr : Intrinsic::fshl, Ty);
return CallInst::Create(OppositeShift, {Op0, Op1, Y});
}

// fshl(X, 0, Y) --> shl(X, and(Y, BitWidth - 1)) if bitwidth is a
// power-of-2
if (IID == Intrinsic::fshl && isPowerOf2_32(BitWidth) &&
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69 changes: 69 additions & 0 deletions llvm/test/Transforms/InstCombine/fsh.ll
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Please also add a negative test where the funnel shift is not a rotate (operands not the same).

Original file line number Diff line number Diff line change
Expand Up @@ -1084,3 +1084,72 @@ define i8 @fshl_range_trunc(i1 %x) {
%tr = trunc nsw i32 %fshl to i8
ret i8 %tr
}

;; Issue #138334 negative rotate amounts can be folded into the opposite direction
define i32 @fshl_neg_amount(i32 %x, i32 %y) {
; CHECK-LABEL: @fshl_neg_amount(
; CHECK-NEXT: [[R:%.*]] = call i32 @llvm.fshr.i32(i32 [[X:%.*]], i32 [[X]], i32 [[Y:%.*]])
; CHECK-NEXT: ret i32 [[R]]
;
%n = sub i32 0, %y
%r = call i32 @llvm.fshl.i32(i32 %x, i32 %x, i32 %n)
ret i32 %r
}

define i32 @fshr_neg_amount(i32 %x, i32 %y) {
; CHECK-LABEL: @fshr_neg_amount(
; CHECK-NEXT: [[R:%.*]] = call i32 @llvm.fshl.i32(i32 [[X:%.*]], i32 [[X]], i32 [[Y:%.*]])
; CHECK-NEXT: ret i32 [[R]]
;
%n = sub i32 0, %y
%r = call i32 @llvm.fshr.i32(i32 %x, i32 %x, i32 %n)
ret i32 %r
}

;; negative test, funnel shift is not a rotate

define i32 @fshl_neg_amount_non_rotate(i32 %x, i32 %y, i32 %z) {
; CHECK-LABEL: @fshl_neg_amount_non_rotate(
; CHECK-NEXT: [[N:%.*]] = sub i32 0, [[Y:%.*]]
; CHECK-NEXT: [[R:%.*]] = call i32 @llvm.fshl.i32(i32 [[X:%.*]], i32 [[Z:%.*]], i32 [[N]])
; CHECK-NEXT: ret i32 [[R]]
;
%n = sub i32 0, %y
%r = call i32 @llvm.fshl.i32(i32 %x, i32 %z, i32 %n)
ret i32 %r
}

define i32 @fshr_neg_amount_non_rotate(i32 %x, i32 %y, i32 %z) {
; CHECK-LABEL: @fshr_neg_amount_non_rotate(
; CHECK-NEXT: [[N:%.*]] = sub i32 0, [[Y:%.*]]
; CHECK-NEXT: [[R:%.*]] = call i32 @llvm.fshr.i32(i32 [[X:%.*]], i32 [[Z:%.*]], i32 [[N]])
; CHECK-NEXT: ret i32 [[R]]
;
%n = sub i32 0, %y
%r = call i32 @llvm.fshr.i32(i32 %x, i32 %z, i32 %n)
ret i32 %r
}

;; negative test, bitwidth is not a power of two

define i31 @fshl_neg_amount_non_power_two(i31 %x, i31 %y) {
; CHECK-LABEL: @fshl_neg_amount_non_power_two(
; CHECK-NEXT: [[N:%.*]] = sub i31 0, [[Y:%.*]]
; CHECK-NEXT: [[R:%.*]] = call i31 @llvm.fshl.i31(i31 [[X:%.*]], i31 [[X]], i31 [[N]])
; CHECK-NEXT: ret i31 [[R]]
;
%n = sub i31 0, %y
%r = call i31 @llvm.fshl.i31(i31 %x, i31 %x, i31 %n)
ret i31 %r
}

define i31 @fshr_neg_amount_non_power_two(i31 %x, i31 %y) {
; CHECK-LABEL: @fshr_neg_amount_non_power_two(
; CHECK-NEXT: [[N:%.*]] = sub i31 0, [[Y:%.*]]
; CHECK-NEXT: [[R:%.*]] = call i31 @llvm.fshr.i31(i31 [[X:%.*]], i31 [[X]], i31 [[N]])
; CHECK-NEXT: ret i31 [[R]]
;
%n = sub i31 0, %y
%r = call i31 @llvm.fshr.i31(i31 %x, i31 %x, i31 %n)
ret i31 %r
}
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