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[InstCombine] eliminate sext and/or trunc if value has enough signbits

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If we have enough signbits in a source value, we can skip an
intermediate cast for a trunc+sext pair:
https://alive2.llvm.org/ce/z/A_mQt-

This is the original problem shown in:
https://llvm.org/PR49543

There's a test that shows we transformed what used to be
a pair of shifts, so that suggests we could add another
ComputeNumSignBits fold starting from a shift.

There does not appear to be any change in compile-time
from the extra analysis:
https://llvm-compile-time-tracker.com/compare.php?from=3d2c9069dcafd0cbb641841aa3dd6e851fb7d760&to=b9513cdf2419704c7bb0c3a02a9ca06aae13d902&stat=instructions

Differential Revision: https://reviews.llvm.org/D103617
This commit is contained in:
Sanjay Patel 2021-06-03 13:15:23 -04:00
parent 40fae0516b
commit 70de13d74d
2 changed files with 28 additions and 22 deletions
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19
lib/Transforms/InstCombine/InstCombineCasts.cpp
View File

@ -1526,13 +1526,20 @@ Instruction *InstCombinerImpl::visitSExt(SExtInst &CI) {
ShAmt);
}
// If the input is a trunc from the destination type, then turn sext(trunc(x))
// into shifts.
Value *X;
if (match(Src, m_OneUse(m_Trunc(m_Value(X)))) && X->getType() == DestTy) {
// sext (trunc X) --> ashr (shl X, C), C
Constant *ShAmt = ConstantInt::get(DestTy, DestBitSize - SrcBitSize);
return BinaryOperator::CreateAShr(Builder.CreateShl(X, ShAmt), ShAmt);
if (match(Src, m_Trunc(m_Value(X)))) {
// If the input has more sign bits than bits truncated, then convert
// directly to final type.
unsigned XBitSize = X->getType()->getScalarSizeInBits();
if (ComputeNumSignBits(X, 0, &CI) > XBitSize - SrcBitSize)
return CastInst::CreateIntegerCast(X, DestTy, /* isSigned */ true);
// If input is a trunc from the destination type, then convert into shifts.
if (Src->hasOneUse() && X->getType() == DestTy) {
// sext (trunc X) --> ashr (shl X, C), C
Constant *ShAmt = ConstantInt::get(DestTy, DestBitSize - SrcBitSize);
return BinaryOperator::CreateAShr(Builder.CreateShl(X, ShAmt), ShAmt);
}
}
if (ICmpInst *ICI = dyn_cast<ICmpInst>(Src))

31
test/Transforms/InstCombine/sext-of-trunc-nsw.ll
View File

@ -13,8 +13,7 @@ define i16 @t0(i8 %x) {
; CHECK-LABEL: @t0(
; CHECK-NEXT: [[A:%.*]] = ashr i8 [[X:%.*]], 5
; CHECK-NEXT: call void @use8(i8 [[A]])
; CHECK-NEXT: [[B:%.*]] = trunc i8 [[A]] to i4
; CHECK-NEXT: [[C:%.*]] = sext i4 [[B]] to i16
; CHECK-NEXT: [[C:%.*]] = sext i8 [[A]] to i16
; CHECK-NEXT: ret i16 [[C]]
;
%a = ashr i8 %x, 5
@ -28,8 +27,7 @@ define i16 @t1(i8 %x) {
; CHECK-LABEL: @t1(
; CHECK-NEXT: [[A:%.*]] = ashr i8 [[X:%.*]], 4
; CHECK-NEXT: call void @use8(i8 [[A]])
; CHECK-NEXT: [[B:%.*]] = trunc i8 [[A]] to i4
; CHECK-NEXT: [[C:%.*]] = sext i4 [[B]] to i16
; CHECK-NEXT: [[C:%.*]] = sext i8 [[A]] to i16
; CHECK-NEXT: ret i16 [[C]]
;
%a = ashr i8 %x, 4
@ -59,8 +57,7 @@ define <2 x i16> @t3_vec(<2 x i8> %x) {
; CHECK-LABEL: @t3_vec(
; CHECK-NEXT: [[A:%.*]] = ashr <2 x i8> [[X:%.*]], <i8 4, i8 4>
; CHECK-NEXT: call void @usevec(<2 x i8> [[A]])
; CHECK-NEXT: [[B:%.*]] = trunc <2 x i8> [[A]] to <2 x i4>
; CHECK-NEXT: [[C:%.*]] = sext <2 x i4> [[B]] to <2 x i16>
; CHECK-NEXT: [[C:%.*]] = sext <2 x i8> [[A]] to <2 x i16>
; CHECK-NEXT: ret <2 x i16> [[C]]
;
%a = ashr <2 x i8> %x, <i8 4, i8 4>
@ -91,7 +88,7 @@ define i16 @t5_extrause(i8 %x) {
; CHECK-NEXT: call void @use8(i8 [[A]])
; CHECK-NEXT: [[B:%.*]] = trunc i8 [[A]] to i4
; CHECK-NEXT: call void @use4(i4 [[B]])
; CHECK-NEXT: [[C:%.*]] = sext i4 [[B]] to i16
; CHECK-NEXT: [[C:%.*]] = sext i8 [[A]] to i16
; CHECK-NEXT: ret i16 [[C]]
;
%a = ashr i8 %x, 5
@ -106,8 +103,7 @@ define i64 @narrow_source_matching_signbits(i32 %x) {
; CHECK-LABEL: @narrow_source_matching_signbits(
; CHECK-NEXT: [[M:%.*]] = and i32 [[X:%.*]], 7
; CHECK-NEXT: [[A:%.*]] = shl nsw i32 -1, [[M]]
; CHECK-NEXT: [[B:%.*]] = trunc i32 [[A]] to i8
; CHECK-NEXT: [[C:%.*]] = sext i8 [[B]] to i64
; CHECK-NEXT: [[C:%.*]] = sext i32 [[A]] to i64
; CHECK-NEXT: ret i64 [[C]]
;
%m = and i32 %x, 7
@ -117,6 +113,8 @@ define i64 @narrow_source_matching_signbits(i32 %x) {
ret i64 %c
}
; negative test - not enough sign-bits
define i64 @narrow_source_not_matching_signbits(i32 %x) {
; CHECK-LABEL: @narrow_source_not_matching_signbits(
; CHECK-NEXT: [[M:%.*]] = and i32 [[X:%.*]], 8
@ -136,8 +134,7 @@ define i24 @wide_source_matching_signbits(i32 %x) {
; CHECK-LABEL: @wide_source_matching_signbits(
; CHECK-NEXT: [[M:%.*]] = and i32 [[X:%.*]], 7
; CHECK-NEXT: [[A:%.*]] = shl nsw i32 -1, [[M]]
; CHECK-NEXT: [[B:%.*]] = trunc i32 [[A]] to i8
; CHECK-NEXT: [[C:%.*]] = sext i8 [[B]] to i24
; CHECK-NEXT: [[C:%.*]] = trunc i32 [[A]] to i24
; CHECK-NEXT: ret i24 [[C]]
;
%m = and i32 %x, 7
@ -147,6 +144,8 @@ define i24 @wide_source_matching_signbits(i32 %x) {
ret i24 %c
}
; negative test - not enough sign-bits
define i24 @wide_source_not_matching_signbits(i32 %x) {
; CHECK-LABEL: @wide_source_not_matching_signbits(
; CHECK-NEXT: [[M2:%.*]] = and i32 [[X:%.*]], 8
@ -165,9 +164,8 @@ define i24 @wide_source_not_matching_signbits(i32 %x) {
define i32 @same_source_matching_signbits(i32 %x) {
; CHECK-LABEL: @same_source_matching_signbits(
; CHECK-NEXT: [[M:%.*]] = and i32 [[X:%.*]], 7
; CHECK-NEXT: [[TMP1:%.*]] = shl i32 -16777216, [[M]]
; CHECK-NEXT: [[C:%.*]] = ashr exact i32 [[TMP1]], 24
; CHECK-NEXT: ret i32 [[C]]
; CHECK-NEXT: [[A:%.*]] = shl nsw i32 -1, [[M]]
; CHECK-NEXT: ret i32 [[A]]
;
%m = and i32 %x, 7
%a = shl nsw i32 -1, %m
@ -176,6 +174,8 @@ define i32 @same_source_matching_signbits(i32 %x) {
ret i32 %c
}
; negative test - not enough sign-bits
define i32 @same_source_not_matching_signbits(i32 %x) {
; CHECK-LABEL: @same_source_not_matching_signbits(
; CHECK-NEXT: [[M2:%.*]] = and i32 [[X:%.*]], 8
@ -196,8 +196,7 @@ define i32 @same_source_matching_signbits_extra_use(i32 %x) {
; CHECK-NEXT: [[A:%.*]] = shl nsw i32 -1, [[M]]
; CHECK-NEXT: [[B:%.*]] = trunc i32 [[A]] to i8
; CHECK-NEXT: call void @use8(i8 [[B]])
; CHECK-NEXT: [[C:%.*]] = sext i8 [[B]] to i32
; CHECK-NEXT: ret i32 [[C]]
; CHECK-NEXT: ret i32 [[A]]
;
%m = and i32 %x, 7
%a = shl nsw i32 -1, %m