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[SCEV] Model ashr exact x, C as (abs(x) EXACT/u (1<<C)) * signum(x)

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It's not pretty, but probably better than modelling it
as an opaque SCEVUnknown, i guess.

It is relevant e.g. for the loop that was brought up in
https://bugs.llvm.org/show_bug.cgi?id=46786#c26
as an example of what we'd be able to better analyze
once SCEV handles `ptrtoint` (D89456).

But as it is evident, even if we deal with `ptrtoint` there,
we also fail to model such an `ashr`.
Also, modeling of mul-of-exact-shr/div could use improvement.

As per alive2:
https://alive2.llvm.org/ce/z/tnfZKd
```
define i8 @src(i8 %0) {
  %2 = ashr exact i8 %0, 4
  ret i8 %2
}

declare i8 @llvm.abs(i8, i1)
declare i8 @llvm.smin(i8, i8)
declare i8 @llvm.smax(i8, i8)

define i8 @tgt(i8 %x) {
  %abs_x = call i8 @llvm.abs(i8 %x, i1 false)
  %div = udiv exact i8 %abs_x, 16
  %t0 = call i8 @llvm.smax(i8 %x, i8 -1)
  %t1 = call i8 @llvm.smin(i8 %t0, i8 1)
  %r = mul nsw i8 %div, %t1
  ret i8 %r
}
```
Transformation seems to be correct!
This commit is contained in:
Roman Lebedev 2020-10-17 21:04:26 +03:00
parent 245883434c
commit 1e8f7ebb6c
4 changed files with 29 additions and 8 deletions
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1
include/llvm/Analysis/ScalarEvolution.h
View File

@ -573,6 +573,7 @@ public:
const SCEV *getGEPExpr(GEPOperator *GEP,
const SmallVectorImpl<const SCEV *> &IndexExprs);
const SCEV *getAbsExpr(const SCEV *Op, bool IsNSW);
const SCEV *getSignumExpr(const SCEV *Op);
const SCEV *getMinMaxExpr(unsigned Kind,
SmallVectorImpl<const SCEV *> &Operands);
const SCEV *getSMaxExpr(const SCEV *LHS, const SCEV *RHS);

22
lib/Analysis/ScalarEvolution.cpp
View File

@ -3339,6 +3339,11 @@ const SCEV *ScalarEvolution::getAbsExpr(const SCEV *Op, bool IsNSW) {
return getSMaxExpr(Op, getNegativeSCEV(Op, Flags));
}
const SCEV *ScalarEvolution::getSignumExpr(const SCEV *Op) {
Type *Ty = Op->getType();
return getSMinExpr(getSMaxExpr(Op, getMinusOne(Ty)), getOne(Ty));
}
const SCEV *ScalarEvolution::getMinMaxExpr(unsigned Kind,
SmallVectorImpl<const SCEV *> &Ops) {
assert(!Ops.empty() && "Cannot get empty (u|s)(min|max)!");
@ -4273,6 +4278,7 @@ struct BinaryOp {
Value *RHS;
bool IsNSW = false;
bool IsNUW = false;
bool IsExact = false;
/// Op is set if this BinaryOp corresponds to a concrete LLVM instruction or
/// constant expression.
@ -4285,11 +4291,14 @@ struct BinaryOp {
IsNSW = OBO->hasNoSignedWrap();
IsNUW = OBO->hasNoUnsignedWrap();
}
if (auto *PEO = dyn_cast<PossiblyExactOperator>(Op))
IsExact = PEO->isExact();
}
explicit BinaryOp(unsigned Opcode, Value *LHS, Value *RHS, bool IsNSW = false,
bool IsNUW = false)
: Opcode(Opcode), LHS(LHS), RHS(RHS), IsNSW(IsNSW), IsNUW(IsNUW) {}
bool IsNUW = false, bool IsExact = false)
: Opcode(Opcode), LHS(LHS), RHS(RHS), IsNSW(IsNSW), IsNUW(IsNUW),
IsExact(IsExact) {}
};
} // end anonymous namespace
@ -6267,6 +6276,15 @@ const SCEV *ScalarEvolution::createSCEV(Value *V) {
}
}
}
if (BO->IsExact) {
// Given exact arithmetic in-bounds right-shift by a constant,
// we can lower it into: (abs(x) EXACT/u (1<<C)) * signum(x)
const SCEV *X = getSCEV(BO->LHS);
const SCEV *AbsX = getAbsExpr(X, /*IsNSW=*/false);
APInt Mult = APInt::getOneBitSet(BitWidth, AShrAmt);
const SCEV *Div = getUDivExactExpr(AbsX, getConstant(Mult));
return getMulExpr(Div, getSignumExpr(X), SCEV::FlagNSW);
}
break;
}
}

4
test/Analysis/ScalarEvolution/ashr.ll
View File

@ -42,7 +42,7 @@ define i32 @t3(i32 %x, i32 %y) {
; ALL-LABEL: 't3'
; ALL-NEXT: Classifying expressions for: @t3
; ALL-NEXT: %i0 = ashr exact i32 %x, 4
; ALL-NEXT: --> %i0 U: full-set S: [-134217728,134217728)
; ALL-NEXT: --> ((((-1 * %x) smax %x) /u 16) * (1 smin (-1 smax %x)))<nsw> U: [-268435455,268435456) S: [-268435455,268435456)
; ALL-NEXT: Determining loop execution counts for: @t3
;
%i0 = ashr exact i32 %x, 4
@ -65,7 +65,7 @@ define i32 @t5(i32 %x, i32 %y) {
; ALL-LABEL: 't5'
; ALL-NEXT: Classifying expressions for: @t5
; ALL-NEXT: %i0 = ashr exact i32 %x, 5
; ALL-NEXT: --> %i0 U: full-set S: [-67108864,67108864)
; ALL-NEXT: --> ((((-1 * %x) smax %x) /u 32) * (1 smin (-1 smax %x)))<nsw> U: [-134217727,134217728) S: [-134217727,134217728)
; ALL-NEXT: Determining loop execution counts for: @t5
;
%i0 = ashr exact i32 %x, 5

10
test/Analysis/ScalarEvolution/ptrtoint.ll
View File

@ -325,6 +325,8 @@ bb5:
; for (int* cur = start; cur != end; ++cur)
; other[cur - start] += *cur;
; }
;
; FIXME: 4 * (%i10 EXACT/s 4) is just %i10
define void @pr46786_c26_int(i32* %arg, i32* %arg1, i32* %arg2) {
; X64-LABEL: 'pr46786_c26_int'
; X64-NEXT: Classifying expressions for: @pr46786_c26_int
@ -339,9 +341,9 @@ define void @pr46786_c26_int(i32* %arg, i32* %arg1, i32* %arg2) {
; X64-NEXT: %i10 = sub i64 %i9, %i4
; X64-NEXT: --> ((-1 * %i4) + %i9) U: full-set S: full-set Exits: <<Unknown>> LoopDispositions: { %bb6: Variant }
; X64-NEXT: %i11 = ashr exact i64 %i10, 2
; X64-NEXT: --> %i11 U: full-set S: [-2305843009213693952,2305843009213693952) Exits: <<Unknown>> LoopDispositions: { %bb6: Variant }
; X64-NEXT: --> (((((-1 * %i4) + %i9) smax ((-1 * %i9) + %i4)) /u 4) * (1 smin (-1 smax ((-1 * %i4) + %i9))))<nsw> U: [-4611686018427387903,4611686018427387904) S: [-4611686018427387903,4611686018427387904) Exits: <<Unknown>> LoopDispositions: { %bb6: Variant }
; X64-NEXT: %i12 = getelementptr inbounds i32, i32* %arg2, i64 %i11
; X64-NEXT: --> ((4 * %i11)<nsw> + %arg2)<nsw> U: full-set S: full-set Exits: <<Unknown>> LoopDispositions: { %bb6: Variant }
; X64-NEXT: --> ((4 * ((((-1 * %i4) + %i9) smax ((-1 * %i9) + %i4)) /u 4) * (1 smin (-1 smax ((-1 * %i4) + %i9)))) + %arg2)<nsw> U: full-set S: full-set Exits: <<Unknown>> LoopDispositions: { %bb6: Variant }
; X64-NEXT: %i13 = load i32, i32* %i12, align 4
; X64-NEXT: --> %i13 U: full-set S: full-set Exits: <<Unknown>> LoopDispositions: { %bb6: Variant }
; X64-NEXT: %i14 = add nsw i32 %i13, %i8
@ -368,9 +370,9 @@ define void @pr46786_c26_int(i32* %arg, i32* %arg1, i32* %arg2) {
; X32-NEXT: %i10 = sub i64 %i9, %i4
; X32-NEXT: --> ((-1 * %i4)<nsw> + %i9) U: [-4294967295,4294967296) S: [-8589934591,8589934592) Exits: <<Unknown>> LoopDispositions: { %bb6: Variant }
; X32-NEXT: %i11 = ashr exact i64 %i10, 2
; X32-NEXT: --> %i11 U: full-set S: [-2147483648,2147483648) Exits: <<Unknown>> LoopDispositions: { %bb6: Variant }
; X32-NEXT: --> (((((-1 * %i4)<nsw> + %i9) smax ((-1 * %i9)<nsw> + %i4)) /u 4) * (1 smin (-1 smax ((-1 * %i4)<nsw> + %i9))))<nsw> U: [-4611686018427387903,4611686018427387904) S: [-4611686018427387903,4611686018427387904) Exits: <<Unknown>> LoopDispositions: { %bb6: Variant }
; X32-NEXT: %i12 = getelementptr inbounds i32, i32* %arg2, i64 %i11
; X32-NEXT: --> ((4 * (trunc i64 %i11 to i32))<nsw> + %arg2)<nsw> U: full-set S: full-set Exits: <<Unknown>> LoopDispositions: { %bb6: Variant }
; X32-NEXT: --> ((4 * (trunc i64 (((((-1 * %i4)<nsw> + %i9) smax ((-1 * %i9)<nsw> + %i4)) /u 4) * (1 smin (-1 smax ((-1 * %i4)<nsw> + %i9))))<nsw> to i32))<nsw> + %arg2)<nsw> U: full-set S: full-set Exits: <<Unknown>> LoopDispositions: { %bb6: Variant }
; X32-NEXT: %i13 = load i32, i32* %i12, align 4
; X32-NEXT: --> %i13 U: full-set S: full-set Exits: <<Unknown>> LoopDispositions: { %bb6: Variant }
; X32-NEXT: %i14 = add nsw i32 %i13, %i8