caio/clang-p2996
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[InstCombine] fold cast of right-shift if high bits are not demanded

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(masked) trunc (lshr X, C) --> (masked) lshr (trunc X), C

Narrowing the shift should be better for analysis and can lead
to follow-on transforms as shown.

Attempt at a general proof in Alive2:
https://alive2.llvm.org/ce/z/tRnnSF

Here are a couple of the specific tests:
https://alive2.llvm.org/ce/z/bCnTp-
https://alive2.llvm.org/ce/z/TfaHnb

Differential Revision: https://reviews.llvm.org/D110170
This commit is contained in:
Sanjay Patel
2021-09-21 15:33:51 -04:00
parent 43d6991c2a
commit 2f6b07316f
2 changed files with 55 additions and 21 deletions
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20
llvm/lib/Transforms/InstCombine/InstCombineSimplifyDemanded.cpp
View File

@@ -385,8 +385,26 @@ Value *InstCombinerImpl::SimplifyDemandedUseBits(Value *V, APInt DemandedMask,
Known = KnownBits::commonBits(LHSKnown, RHSKnown);
break;
}
case Instruction::ZExt:
case Instruction::Trunc: {
// If we do not demand the high bits of a right-shifted and truncated value,
// then we may be able to truncate it before the shift.
Value *X;
const APInt *C;
if (match(I->getOperand(0), m_OneUse(m_LShr(m_Value(X), m_APInt(C))))) {
// The shift amount must be valid (not poison) in the narrow type, and
// it must not be greater than the high bits demanded of the result.
if (C->ult(I->getType()->getScalarSizeInBits()) &&
C->ule(DemandedMask.countLeadingZeros())) {
// trunc (lshr X, C) --> lshr (trunc X), C
IRBuilderBase::InsertPointGuard Guard(Builder);
Builder.SetInsertPoint(I);
Value *Trunc = Builder.CreateTrunc(X, I->getType());
return Builder.CreateLShr(Trunc, C->getZExtValue());
}
}
}
LLVM_FALLTHROUGH;
case Instruction::ZExt: {
unsigned SrcBitWidth = I->getOperand(0)->getType()->getScalarSizeInBits();
APInt InputDemandedMask = DemandedMask.zextOrTrunc(SrcBitWidth);

56
llvm/test/Transforms/InstCombine/trunc-demand.ll
View File

@@ -6,9 +6,9 @@ declare void @use8(i8)
define i6 @trunc_lshr(i8 %x) {
; CHECK-LABEL: @trunc_lshr(
; CHECK-NEXT: [[S:%.*]] = lshr i8 [[X:%.*]], 2
; CHECK-NEXT: [[T:%.*]] = trunc i8 [[S]] to i6
; CHECK-NEXT: [[R:%.*]] = and i6 [[T]], 14
; CHECK-NEXT: [[TMP1:%.*]] = trunc i8 [[X:%.*]] to i6
; CHECK-NEXT: [[TMP2:%.*]] = lshr i6 [[TMP1]], 2
; CHECK-NEXT: [[R:%.*]] = and i6 [[TMP2]], 14
; CHECK-NEXT: ret i6 [[R]]
;
%s = lshr i8 %x, 2
@@ -17,12 +17,13 @@ define i6 @trunc_lshr(i8 %x) {
ret i6 %r
}
; The 'and' is eliminated.
define i6 @trunc_lshr_exact_mask(i8 %x) {
; CHECK-LABEL: @trunc_lshr_exact_mask(
; CHECK-NEXT: [[S:%.*]] = lshr i8 [[X:%.*]], 2
; CHECK-NEXT: [[T:%.*]] = trunc i8 [[S]] to i6
; CHECK-NEXT: [[R:%.*]] = and i6 [[T]], 15
; CHECK-NEXT: ret i6 [[R]]
; CHECK-NEXT: [[TMP1:%.*]] = trunc i8 [[X:%.*]] to i6
; CHECK-NEXT: [[TMP2:%.*]] = lshr i6 [[TMP1]], 2
; CHECK-NEXT: ret i6 [[TMP2]]
;
%s = lshr i8 %x, 2
%t = trunc i8 %s to i6
@@ -30,6 +31,8 @@ define i6 @trunc_lshr_exact_mask(i8 %x) {
ret i6 %r
}
; negative test - a high bit of x is in the result
define i6 @trunc_lshr_big_mask(i8 %x) {
; CHECK-LABEL: @trunc_lshr_big_mask(
; CHECK-NEXT: [[S:%.*]] = lshr i8 [[X:%.*]], 2
@@ -43,6 +46,8 @@ define i6 @trunc_lshr_big_mask(i8 %x) {
ret i6 %r
}
; negative test - too many uses
define i6 @trunc_lshr_use1(i8 %x) {
; CHECK-LABEL: @trunc_lshr_use1(
; CHECK-NEXT: [[S:%.*]] = lshr i8 [[X:%.*]], 2
@@ -58,6 +63,8 @@ define i6 @trunc_lshr_use1(i8 %x) {
ret i6 %r
}
; negative test - too many uses
define i6 @trunc_lshr_use2(i8 %x) {
; CHECK-LABEL: @trunc_lshr_use2(
; CHECK-NEXT: [[S:%.*]] = lshr i8 [[X:%.*]], 2
@@ -73,11 +80,13 @@ define i6 @trunc_lshr_use2(i8 %x) {
ret i6 %r
}
; Splat vectors are ok.
define <2 x i7> @trunc_lshr_vec_splat(<2 x i16> %x) {
; CHECK-LABEL: @trunc_lshr_vec_splat(
; CHECK-NEXT: [[S:%.*]] = lshr <2 x i16> [[X:%.*]], <i16 5, i16 5>
; CHECK-NEXT: [[T:%.*]] = trunc <2 x i16> [[S]] to <2 x i7>
; CHECK-NEXT: [[R:%.*]] = and <2 x i7> [[T]], <i7 1, i7 1>
; CHECK-NEXT: [[TMP1:%.*]] = trunc <2 x i16> [[X:%.*]] to <2 x i7>
; CHECK-NEXT: [[TMP2:%.*]] = lshr <2 x i7> [[TMP1]], <i7 5, i7 5>
; CHECK-NEXT: [[R:%.*]] = and <2 x i7> [[TMP2]], <i7 1, i7 1>
; CHECK-NEXT: ret <2 x i7> [[R]]
;
%s = lshr <2 x i16> %x, <i16 5, i16 5>
@@ -86,12 +95,13 @@ define <2 x i7> @trunc_lshr_vec_splat(<2 x i16> %x) {
ret <2 x i7> %r
}
; The 'and' is eliminated.
define <2 x i7> @trunc_lshr_vec_splat_exact_mask(<2 x i16> %x) {
; CHECK-LABEL: @trunc_lshr_vec_splat_exact_mask(
; CHECK-NEXT: [[S:%.*]] = lshr <2 x i16> [[X:%.*]], <i16 6, i16 6>
; CHECK-NEXT: [[T:%.*]] = trunc <2 x i16> [[S]] to <2 x i7>
; CHECK-NEXT: [[R:%.*]] = and <2 x i7> [[T]], <i7 1, i7 1>
; CHECK-NEXT: ret <2 x i7> [[R]]
; CHECK-NEXT: [[TMP1:%.*]] = trunc <2 x i16> [[X:%.*]] to <2 x i7>
; CHECK-NEXT: [[TMP2:%.*]] = lshr <2 x i7> [[TMP1]], <i7 6, i7 6>
; CHECK-NEXT: ret <2 x i7> [[TMP2]]
;
%s = lshr <2 x i16> %x, <i16 6, i16 6>
%t = trunc <2 x i16> %s to <2 x i7>
@@ -99,6 +109,8 @@ define <2 x i7> @trunc_lshr_vec_splat_exact_mask(<2 x i16> %x) {
ret <2 x i7> %r
}
; negative test - the shift is too big for the narrow type
define <2 x i7> @trunc_lshr_big_shift(<2 x i16> %x) {
; CHECK-LABEL: @trunc_lshr_big_shift(
; CHECK-NEXT: [[S:%.*]] = lshr <2 x i16> [[X:%.*]], <i16 7, i16 7>
@@ -112,11 +124,13 @@ define <2 x i7> @trunc_lshr_big_shift(<2 x i16> %x) {
ret <2 x i7> %r
}
; High bits could also be set rather than cleared.
define i6 @or_trunc_lshr(i8 %x) {
; CHECK-LABEL: @or_trunc_lshr(
; CHECK-NEXT: [[S:%.*]] = lshr i8 [[X:%.*]], 1
; CHECK-NEXT: [[T:%.*]] = trunc i8 [[S]] to i6
; CHECK-NEXT: [[R:%.*]] = or i6 [[T]], -32
; CHECK-NEXT: [[TMP1:%.*]] = trunc i8 [[X:%.*]] to i6
; CHECK-NEXT: [[TMP2:%.*]] = lshr i6 [[TMP1]], 1
; CHECK-NEXT: [[R:%.*]] = or i6 [[TMP2]], -32
; CHECK-NEXT: ret i6 [[R]]
;
%s = lshr i8 %x, 1
@@ -127,9 +141,9 @@ define i6 @or_trunc_lshr(i8 %x) {
define i6 @or_trunc_lshr_more(i8 %x) {
; CHECK-LABEL: @or_trunc_lshr_more(
; CHECK-NEXT: [[S:%.*]] = lshr i8 [[X:%.*]], 4
; CHECK-NEXT: [[T:%.*]] = trunc i8 [[S]] to i6
; CHECK-NEXT: [[R:%.*]] = or i6 [[T]], -4
; CHECK-NEXT: [[TMP1:%.*]] = trunc i8 [[X:%.*]] to i6
; CHECK-NEXT: [[TMP2:%.*]] = lshr i6 [[TMP1]], 4
; CHECK-NEXT: [[R:%.*]] = or i6 [[TMP2]], -4
; CHECK-NEXT: ret i6 [[R]]
;
%s = lshr i8 %x, 4
@@ -138,6 +152,8 @@ define i6 @or_trunc_lshr_more(i8 %x) {
ret i6 %r
}
; negative test - need all high bits to be undemanded
define i6 @or_trunc_lshr_small_mask(i8 %x) {
; CHECK-LABEL: @or_trunc_lshr_small_mask(
; CHECK-NEXT: [[S:%.*]] = lshr i8 [[X:%.*]], 4