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When factoring multiply expressions across adds, factor both

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positive and negative forms of constants together.  This 
allows us to compile:

int foo(int x, int y) {
    return (x-y) + (x-y) + (x-y);
}

into:

_foo:                                                       ## @foo
	subl	%esi, %edi
	leal	(%rdi,%rdi,2), %eax
	ret

instead of (where the 3 and -3 were not factored):

_foo:
        imull   $-3, 8(%esp), %ecx
        imull   $3, 4(%esp), %eax
        addl    %ecx, %eax
        ret

this started out as:
    movl    12(%ebp), %ecx
    imull   $3, 8(%ebp), %eax
    subl    %ecx, %eax
    subl    %ecx, %eax
    subl    %ecx, %eax
    ret

This comes from PR5359.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@92381 91177308-0d34-0410-b5e6-96231b3b80d8
This commit is contained in:
Chris Lattner 2010-01-01 01:13:15 +00:00
parent 75954e0bbd
commit 9506c930aa
2 changed files with 62 additions and 21 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

70
lib/Transforms/Scalar/Reassociate.cpp
View File

@ -510,7 +510,8 @@ static Instruction *ConvertShiftToMul(Instruction *Shl,
}
// Scan backwards and forwards among values with the same rank as element i to
// see if X exists. If X does not exist, return i.
// see if X exists. If X does not exist, return i. This is useful when
// scanning for 'x' when we see '-x' because they both get the same rank.
static unsigned FindInOperandList(SmallVectorImpl<ValueEntry> &Ops, unsigned i,
Value *X) {
unsigned XRank = Ops[i].Rank;
@ -518,7 +519,7 @@ static unsigned FindInOperandList(SmallVectorImpl<ValueEntry> &Ops, unsigned i,
for (unsigned j = i+1; j != e && Ops[j].Rank == XRank; ++j)
if (Ops[j].Op == X)
return j;
// Scan backwards
// Scan backwards.
for (unsigned j = i-1; j != ~0U && Ops[j].Rank == XRank; --j)
if (Ops[j].Op == X)
return j;
@ -547,28 +548,47 @@ Value *Reassociate::RemoveFactorFromExpression(Value *V, Value *Factor) {
LinearizeExprTree(BO, Factors);
bool FoundFactor = false;
for (unsigned i = 0, e = Factors.size(); i != e; ++i)
bool NeedsNegate = false;
for (unsigned i = 0, e = Factors.size(); i != e; ++i) {
if (Factors[i].Op == Factor) {
FoundFactor = true;
Factors.erase(Factors.begin()+i);
break;
}
// If this is a negative version of this factor, remove it.
if (ConstantInt *FC1 = dyn_cast<ConstantInt>(Factor))
if (ConstantInt *FC2 = dyn_cast<ConstantInt>(Factors[i].Op))
if (FC1->getValue() == -FC2->getValue()) {
FoundFactor = NeedsNegate = true;
Factors.erase(Factors.begin()+i);
break;
}
}
if (!FoundFactor) {
// Make sure to restore the operands to the expression tree.
RewriteExprTree(BO, Factors);
return 0;
}
BasicBlock::iterator InsertPt = BO; ++InsertPt;
// If this was just a single multiply, remove the multiply and return the only
// remaining operand.
if (Factors.size() == 1) {
ValueRankMap.erase(BO);
BO->eraseFromParent();
return Factors[0].Op;
V = Factors[0].Op;
} else {
RewriteExprTree(BO, Factors);
V = BO;
}
RewriteExprTree(BO, Factors);
return BO;
if (NeedsNegate)
V = BinaryOperator::CreateNeg(V, "neg", InsertPt);
return V;
}
/// FindSingleUseMultiplyFactors - If V is a single-use multiply, recursively
@ -645,6 +665,9 @@ Value *Reassociate::OptimizeAdd(Instruction *I,
// Scan the operand lists looking for X and -X pairs. If we find any, we
// can simplify the expression. X+-X == 0. While we're at it, scan for any
// duplicates. We want to canonicalize Y+Y+Y+Z -> 3*Y+Z.
//
// TODO: We could handle "X + ~X" -> "-1" if we wanted, since "-X = ~X+1".
//
for (unsigned i = 0, e = Ops.size(); i != e; ++i) {
Value *TheOp = Ops[i].Op;
// Check to see if we've seen this operand before. If so, we factor all
@ -730,21 +753,26 @@ Value *Reassociate::OptimizeAdd(Instruction *I,
assert(Factors.size() > 1 && "Bad linearize!");
// Add one to FactorOccurrences for each unique factor in this op.
if (Factors.size() == 2) {
unsigned Occ = ++FactorOccurrences[Factors[0]];
if (Occ > MaxOcc) { MaxOcc = Occ; MaxOccVal = Factors[0]; }
if (Factors[0] != Factors[1]) { // Don't double count A*A.
Occ = ++FactorOccurrences[Factors[1]];
if (Occ > MaxOcc) { MaxOcc = Occ; MaxOccVal = Factors[1]; }
}
} else {
SmallPtrSet<Value*, 4> Duplicates;
for (unsigned i = 0, e = Factors.size(); i != e; ++i) {
if (!Duplicates.insert(Factors[i])) continue;
unsigned Occ = ++FactorOccurrences[Factors[i]];
if (Occ > MaxOcc) { MaxOcc = Occ; MaxOccVal = Factors[i]; }
}
SmallPtrSet<Value*, 8> Duplicates;
for (unsigned i = 0, e = Factors.size(); i != e; ++i) {
Value *Factor = Factors[i];
if (!Duplicates.insert(Factor)) continue;
unsigned Occ = ++FactorOccurrences[Factor];
if (Occ > MaxOcc) { MaxOcc = Occ; MaxOccVal = Factor; }
// If Factor is a negative constant, add the negated value as a factor
// because we can percolate the negate out. Watch for minint, which
// cannot be positivified.
if (ConstantInt *CI = dyn_cast<ConstantInt>(Factor))
if (CI->getValue().isNegative() && !CI->getValue().isMinSignedValue()) {
Factor = ConstantInt::get(CI->getContext(), -CI->getValue());
assert(!Duplicates.count(Factor) &&
"Shouldn't have two constant factors, missed a canonicalize");
unsigned Occ = ++FactorOccurrences[Factor];
if (Occ > MaxOcc) { MaxOcc = Occ; MaxOccVal = Factor; }
}
}
}

13
test/Transforms/Reassociate/basictest.ll
View File

@ -191,3 +191,16 @@ define i32 @test13(i32 %X1, i32 %X2, i32 %X3) {
; CHECK-NEXT: ret i32
}
; PR5359
define i32 @test14(i32 %X1, i32 %X2) {
%B = mul i32 %X1, 47 ; X1*47
%C = mul i32 %X2, -47 ; X2*-47
%D = add i32 %B, %C ; X1*47 + X2*-47 -> 47*(X1-X2)
ret i32 %D
; CHECK: @test14
; CHECK-NEXT: sub i32 %X1, %X2
; CHECK-NEXT: mul i32 {{.*}}, 47
; CHECK-NEXT: ret i32
}