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[InstCombine] Shift amount reassociation in shifty sign bit test (PR43595)

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Summary:
This problem consists of several parts:
* Basic sign bit extraction - `trunc? (?shr %x, (bitwidth(x)-1))`.
  This is trivial, and easy to do, we have a fold for it.
* Shift amount reassociation - if we have two identical shifts,
  and we can simplify-add their shift amounts together,
  then we likely can just perform them as a single shift.
  But this is finicky, has one-use restrictions,
  and shift opcodes must be identical.

But there is a super-pattern where both of these work together.
to produce sign bit test from two shifts + comparison.
We do indeed already handle this in most cases.
But since we get that fold transitively, it has one-use restrictions.
And what's worse, in this case the right-shifts aren't required to be
identical, and we can't handle that transitively:

If the total shift amount is bitwidth-1, only a sign bit will remain
in the output value. But if we look at this from the perspective of
two shifts, we can't fold - we can't possibly know what bit pattern
we'd produce via two shifts, it will be *some* kind of a mask
produced from original sign bit, but we just can't tell it's shape:
https://rise4fun.com/Alive/cM0 https://rise4fun.com/Alive/9IN

But it will *only* contain sign bit and zeros.
So from the perspective of sign bit test, we're good:
https://rise4fun.com/Alive/FRz https://rise4fun.com/Alive/qBU
Superb!

So the simplest solution is to extend `reassociateShiftAmtsOfTwoSameDirectionShifts()` to also have a
sudo-analysis mode that will ignore extra-uses, and will only check
whether a) those are two right shifts and b) they end up with bitwidth(x)-1
shift amount and return either the original value that we sign-checking,
or null.

This does not have any functionality change for
the existing `reassociateShiftAmtsOfTwoSameDirectionShifts()`.

All that being said, as disscussed in the review, this yet again
increases usage of instsimplify in instcombine as utility.
Some day that may need to be reevaluated.

https://bugs.llvm.org/show_bug.cgi?id=43595

Reviewers: spatel, efriedma, vsk

Reviewed By: spatel

Subscribers: xbolva00, hiraditya, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D68930

llvm-svn: 375371
This commit is contained in:
Roman Lebedev 2019-10-20 19:38:50 +00:00
parent 434e59f951
commit 4583103460
4 changed files with 66 additions and 32 deletions
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29
lib/Transforms/InstCombine/InstCombineCompares.cpp
View File

@ -1358,19 +1358,28 @@ Instruction *InstCombiner::foldIRemByPowerOfTwoToBitTest(ICmpInst &I) {
/// Fold equality-comparison between zero and any (maybe truncated) right-shift
/// by one-less-than-bitwidth into a sign test on the original value.
Instruction *foldSignBitTest(ICmpInst &I) {
Instruction *InstCombiner::foldSignBitTest(ICmpInst &I) {
Instruction *Val;
ICmpInst::Predicate Pred;
Value *X;
Constant *C;
if (!I.isEquality() ||
!match(&I, m_ICmp(Pred, m_TruncOrSelf(m_Shr(m_Value(X), m_Constant(C))),
m_Zero())))
if (!I.isEquality() || !match(&I, m_ICmp(Pred, m_Instruction(Val), m_Zero())))
return nullptr;
Type *XTy = X->getType();
unsigned XBitWidth = XTy->getScalarSizeInBits();
if (!match(C, m_SpecificInt_ICMP(ICmpInst::Predicate::ICMP_EQ,
APInt(XBitWidth, XBitWidth - 1))))
Value *X;
Type *XTy;
Constant *C;
if (match(Val, m_TruncOrSelf(m_Shr(m_Value(X), m_Constant(C))))) {
XTy = X->getType();
unsigned XBitWidth = XTy->getScalarSizeInBits();
if (!match(C, m_SpecificInt_ICMP(ICmpInst::Predicate::ICMP_EQ,
APInt(XBitWidth, XBitWidth - 1))))
return nullptr;
} else if (isa<BinaryOperator>(Val) &&
(X = reassociateShiftAmtsOfTwoSameDirectionShifts(
cast<BinaryOperator>(Val), SQ.getWithInstruction(Val),
/*AnalyzeForSignBitExtraction=*/true))) {
XTy = X->getType();
} else
return nullptr;
return ICmpInst::Create(Instruction::ICmp,

4
lib/Transforms/InstCombine/InstCombineInternal.h
View File

@ -390,6 +390,9 @@ public:
Instruction *visitOr(BinaryOperator &I);
Instruction *visitXor(BinaryOperator &I);
Instruction *visitShl(BinaryOperator &I);
Value *reassociateShiftAmtsOfTwoSameDirectionShifts(
BinaryOperator *Sh0, const SimplifyQuery &SQ,
bool AnalyzeForSignBitExtraction = false);
Instruction *foldVariableSignZeroExtensionOfVariableHighBitExtract(
BinaryOperator &OldAShr);
Instruction *visitAShr(BinaryOperator &I);
@ -912,6 +915,7 @@ private:
Instruction *foldICmpBinOp(ICmpInst &Cmp, const SimplifyQuery &SQ);
Instruction *foldICmpEquality(ICmpInst &Cmp);
Instruction *foldIRemByPowerOfTwoToBitTest(ICmpInst &I);
Instruction *foldSignBitTest(ICmpInst &I);
Instruction *foldICmpWithZero(ICmpInst &Cmp);
Value *foldUnsignedMultiplicationOverflowCheck(ICmpInst &Cmp);

53
lib/Transforms/InstCombine/InstCombineShifts.cpp
View File

@ -25,10 +25,12 @@ using namespace PatternMatch;
// we should rewrite it as
// x shiftopcode (Q+K) iff (Q+K) u< bitwidth(x)
// This is valid for any shift, but they must be identical.
static Instruction *
reassociateShiftAmtsOfTwoSameDirectionShifts(BinaryOperator *Sh0,
const SimplifyQuery &SQ,
InstCombiner::BuilderTy &Builder) {
//
// AnalyzeForSignBitExtraction indicates that we will only analyze whether this
// pattern has any 2 right-shifts that sum to 1 less than original bit width.
Value *InstCombiner::reassociateShiftAmtsOfTwoSameDirectionShifts(
BinaryOperator *Sh0, const SimplifyQuery &SQ,
bool AnalyzeForSignBitExtraction) {
// Look for a shift of some instruction, ignore zext of shift amount if any.
Instruction *Sh0Op0;
Value *ShAmt0;
@ -56,14 +58,25 @@ reassociateShiftAmtsOfTwoSameDirectionShifts(BinaryOperator *Sh0,
if (ShAmt0->getType() != ShAmt1->getType())
return nullptr;
// The shift opcodes must be identical.
// We are only looking for signbit extraction if we have two right shifts.
bool HadTwoRightShifts = match(Sh0, m_Shr(m_Value(), m_Value())) &&
match(Sh1, m_Shr(m_Value(), m_Value()));
// ... and if it's not two right-shifts, we know the answer already.
if (AnalyzeForSignBitExtraction && !HadTwoRightShifts)
return nullptr;
// The shift opcodes must be identical, unless we are just checking whether
// this pattern can be interpreted as a sign-bit-extraction.
Instruction::BinaryOps ShiftOpcode = Sh0->getOpcode();
if (ShiftOpcode != Sh1->getOpcode())
bool IdenticalShOpcodes = Sh0->getOpcode() == Sh1->getOpcode();
if (!IdenticalShOpcodes && !AnalyzeForSignBitExtraction)
return nullptr;
// If we saw truncation, we'll need to produce extra instruction,
// and for that one of the operands of the shift must be one-use.
if (Trunc && !match(Sh0, m_c_BinOp(m_OneUse(m_Value()), m_Value())))
// and for that one of the operands of the shift must be one-use,
// unless of course we don't actually plan to produce any instructions here.
if (Trunc && !AnalyzeForSignBitExtraction &&
!match(Sh0, m_c_BinOp(m_OneUse(m_Value()), m_Value())))
return nullptr;
// Can we fold (ShAmt0+ShAmt1) ?
@ -80,14 +93,22 @@ reassociateShiftAmtsOfTwoSameDirectionShifts(BinaryOperator *Sh0,
return nullptr; // FIXME: could perform constant-folding.
// If there was a truncation, and we have a right-shift, we can only fold if
// we are left with the original sign bit.
// we are left with the original sign bit. Likewise, if we were just checking
// that this is a sighbit extraction, this is the place to check it.
// FIXME: zero shift amount is also legal here, but we can't *easily* check
// more than one predicate so it's not really worth it.
if (Trunc && ShiftOpcode != Instruction::BinaryOps::Shl &&
!match(NewShAmt,
m_SpecificInt_ICMP(ICmpInst::Predicate::ICMP_EQ,
APInt(NewShAmtBitWidth, XBitWidth - 1))))
return nullptr;
if (HadTwoRightShifts && (Trunc || AnalyzeForSignBitExtraction)) {
// If it's not a sign bit extraction, then we're done.
if (!match(NewShAmt,
m_SpecificInt_ICMP(ICmpInst::Predicate::ICMP_EQ,
APInt(NewShAmtBitWidth, XBitWidth - 1))))
return nullptr;
// If it is, and that was the question, return the base value.
if (AnalyzeForSignBitExtraction)
return X;
}
assert(IdenticalShOpcodes && "Should not get here with different shifts.");
// All good, we can do this fold.
NewShAmt = ConstantExpr::getZExtOrBitCast(NewShAmt, X->getType());
@ -287,8 +308,8 @@ Instruction *InstCombiner::commonShiftTransforms(BinaryOperator &I) {
if (Instruction *Res = FoldShiftByConstant(Op0, CUI, I))
return Res;
if (Instruction *NewShift =
reassociateShiftAmtsOfTwoSameDirectionShifts(&I, SQ, Builder))
if (auto *NewShift = cast_or_null<Instruction>(
reassociateShiftAmtsOfTwoSameDirectionShifts(&I, SQ)))
return NewShift;
// (C1 shift (A add C2)) -> (C1 shift C2) shift A)

12
test/Transforms/InstCombine/sign-bit-test-via-right-shifting-all-other-bits.ll
View File

@ -45,7 +45,7 @@ define i1 @highest_bit_test_via_lshr_with_truncation(i64 %data, i32 %nbits) {
; CHECK-NEXT: call void @use32(i32 [[HIGH_BITS_EXTRACTED_NARROW]])
; CHECK-NEXT: call void @use32(i32 [[SKIP_ALL_BITS_TILL_SIGNBIT]])
; CHECK-NEXT: call void @use32(i32 [[SIGNBIT]])
; CHECK-NEXT: [[ISNEG:%.*]] = icmp ne i32 [[SIGNBIT]], 0
; CHECK-NEXT: [[ISNEG:%.*]] = icmp slt i64 [[DATA]], 0
; CHECK-NEXT: ret i1 [[ISNEG]]
;
%num_low_bits_to_skip = sub i32 64, %nbits
@ -107,7 +107,7 @@ define i1 @highest_bit_test_via_ashr_with_truncation(i64 %data, i32 %nbits) {
; CHECK-NEXT: call void @use32(i32 [[HIGH_BITS_EXTRACTED_NARROW]])
; CHECK-NEXT: call void @use32(i32 [[SKIP_ALL_BITS_TILL_SIGNBIT]])
; CHECK-NEXT: call void @use32(i32 [[SIGNBIT]])
; CHECK-NEXT: [[ISNEG:%.*]] = icmp ne i32 [[SIGNBIT]], 0
; CHECK-NEXT: [[ISNEG:%.*]] = icmp slt i64 [[DATA]], 0
; CHECK-NEXT: ret i1 [[ISNEG]]
;
%num_low_bits_to_skip = sub i32 64, %nbits
@ -138,7 +138,7 @@ define i1 @highest_bit_test_via_lshr_ashr(i32 %data, i32 %nbits) {
; CHECK-NEXT: call void @use32(i32 [[HIGH_BITS_EXTRACTED]])
; CHECK-NEXT: call void @use32(i32 [[SKIP_ALL_BITS_TILL_SIGNBIT]])
; CHECK-NEXT: call void @use32(i32 [[SIGNBIT]])
; CHECK-NEXT: [[ISNEG:%.*]] = icmp ne i32 [[SIGNBIT]], 0
; CHECK-NEXT: [[ISNEG:%.*]] = icmp slt i32 [[DATA]], 0
; CHECK-NEXT: ret i1 [[ISNEG]]
;
%num_low_bits_to_skip = sub i32 32, %nbits
@ -169,7 +169,7 @@ define i1 @highest_bit_test_via_lshr_ashe_with_truncation(i64 %data, i32 %nbits)
; CHECK-NEXT: call void @use32(i32 [[HIGH_BITS_EXTRACTED_NARROW]])
; CHECK-NEXT: call void @use32(i32 [[SKIP_ALL_BITS_TILL_SIGNBIT]])
; CHECK-NEXT: call void @use32(i32 [[SIGNBIT]])
; CHECK-NEXT: [[ISNEG:%.*]] = icmp ne i32 [[SIGNBIT]], 0
; CHECK-NEXT: [[ISNEG:%.*]] = icmp slt i64 [[DATA]], 0
; CHECK-NEXT: ret i1 [[ISNEG]]
;
%num_low_bits_to_skip = sub i32 64, %nbits
@ -200,7 +200,7 @@ define i1 @highest_bit_test_via_ashr_lshr(i32 %data, i32 %nbits) {
; CHECK-NEXT: call void @use32(i32 [[HIGH_BITS_EXTRACTED]])
; CHECK-NEXT: call void @use32(i32 [[SKIP_ALL_BITS_TILL_SIGNBIT]])
; CHECK-NEXT: call void @use32(i32 [[SIGNBIT]])
; CHECK-NEXT: [[ISNEG:%.*]] = icmp ne i32 [[SIGNBIT]], 0
; CHECK-NEXT: [[ISNEG:%.*]] = icmp slt i32 [[DATA]], 0
; CHECK-NEXT: ret i1 [[ISNEG]]
;
%num_low_bits_to_skip = sub i32 32, %nbits
@ -231,7 +231,7 @@ define i1 @highest_bit_test_via_ashr_lshr_with_truncation(i64 %data, i32 %nbits)
; CHECK-NEXT: call void @use32(i32 [[HIGH_BITS_EXTRACTED_NARROW]])
; CHECK-NEXT: call void @use32(i32 [[SKIP_ALL_BITS_TILL_SIGNBIT]])
; CHECK-NEXT: call void @use32(i32 [[SIGNBIT]])
; CHECK-NEXT: [[ISNEG:%.*]] = icmp ne i32 [[SIGNBIT]], 0
; CHECK-NEXT: [[ISNEG:%.*]] = icmp slt i64 [[DATA]], 0
; CHECK-NEXT: ret i1 [[ISNEG]]
;
%num_low_bits_to_skip = sub i32 64, %nbits