InstCombine: Be more agressive optimizing 'udiv' instrs with 'select' denoms
Real world code sometimes has the denominator of a 'udiv' be a
'select'. LLVM can handle such cases but only when the 'select'
operands are symmetric in structure (both select operands are a constant
power of two or a left shift, etc.). This falls apart if we are dealt a
'udiv' where the code is not symetric or if the select operands lead us
to more select instructions.
Instead, we should treat the LHS and each select operand as a distinct
divide operation and try to optimize them independently. If we can
to simplify each operation, then we can replace the 'udiv' with, say, a
'lshr' that has a new select with a bunch of new operands for the
select.
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@185257 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/lib/Transforms/InstCombine/InstCombineMulDivRem.cpp b/lib/Transforms/InstCombine/InstCombineMulDivRem.cpp
index d0d4f41..bc5d699 100644
--- a/lib/Transforms/InstCombine/InstCombineMulDivRem.cpp
+++ b/lib/Transforms/InstCombine/InstCombineMulDivRem.cpp
@@ -705,6 +705,80 @@
return 0;
}
+const unsigned MaxDepth = 6;
+
+// \brief Recursively visits the possible right hand operands of a udiv
+// instruction, seeing through select instructions, to determine if we can
+// replace the udiv with something simpler. If we find that an operand is not
+// able to simplify the udiv, we abort the entire transformation.
+//
+// Inserts any intermediate instructions used for the simplification into
+// NewInstrs and returns a new instruction that depends upon them.
+static Instruction *visitUDivOperand(Value *Op0, Value *Op1,
+ const BinaryOperator &I,
+ SmallVectorImpl<Instruction *> &NewInstrs,
+ unsigned Depth = 0) {
+ {
+ // X udiv 2^C -> X >> C
+ // Check to see if this is an unsigned division with an exact power of 2,
+ // if so, convert to a right shift.
+ const APInt *C;
+ if (match(Op1, m_Power2(C))) {
+ BinaryOperator *LShr = BinaryOperator::CreateLShr(
+ Op0, ConstantInt::get(Op0->getType(), C->logBase2()));
+ if (I.isExact()) LShr->setIsExact();
+ return LShr;
+ }
+ }
+
+ if (ConstantInt *C = dyn_cast<ConstantInt>(Op1)) {
+ // X udiv C, where C >= signbit
+ if (C->getValue().isNegative()) {
+ ICmpInst *IC = new ICmpInst(ICmpInst::ICMP_ULT, Op0, C);
+ NewInstrs.push_back(IC);
+
+ return SelectInst::Create(IC, Constant::getNullValue(I.getType()),
+ ConstantInt::get(I.getType(), 1));
+ }
+ }
+
+ // X udiv (C1 << N), where C1 is "1<<C2" --> X >> (N+C2)
+ { const APInt *CI; Value *N;
+ if (match(Op1, m_Shl(m_Power2(CI), m_Value(N))) ||
+ match(Op1, m_ZExt(m_Shl(m_Power2(CI), m_Value(N))))) {
+ if (*CI != 1) {
+ N = BinaryOperator::CreateAdd(
+ N, ConstantInt::get(N->getType(), CI->logBase2()));
+ NewInstrs.push_back(cast<Instruction>(N));
+ }
+ if (ZExtInst *Z = dyn_cast<ZExtInst>(Op1)) {
+ N = new ZExtInst(N, Z->getDestTy());
+ NewInstrs.push_back(cast<Instruction>(N));
+ }
+ BinaryOperator *LShr = BinaryOperator::CreateLShr(Op0, N);
+ if (I.isExact()) LShr->setIsExact();
+ return LShr;
+ }
+ }
+
+ // The remaining tests are all recursive, so bail out if we hit the limit.
+ if (Depth++ == MaxDepth)
+ return 0;
+
+ if (SelectInst *SI = dyn_cast<SelectInst>(Op1))
+ if (Instruction *LHS =
+ visitUDivOperand(Op0, SI->getOperand(1), I, NewInstrs)) {
+ NewInstrs.push_back(LHS);
+ if (Instruction *RHS =
+ visitUDivOperand(Op0, SI->getOperand(2), I, NewInstrs)) {
+ NewInstrs.push_back(RHS);
+ return SelectInst::Create(SI->getCondition(), LHS, RHS);
+ }
+ }
+
+ return 0;
+}
+
Instruction *InstCombiner::visitUDiv(BinaryOperator &I) {
Value *Op0 = I.getOperand(0), *Op1 = I.getOperand(1);
@@ -715,30 +789,6 @@
if (Instruction *Common = commonIDivTransforms(I))
return Common;
- {
- // X udiv 2^C -> X >> C
- // Check to see if this is an unsigned division with an exact power of 2,
- // if so, convert to a right shift.
- const APInt *C;
- if (match(Op1, m_Power2(C))) {
- BinaryOperator *LShr =
- BinaryOperator::CreateLShr(Op0,
- ConstantInt::get(Op0->getType(),
- C->logBase2()));
- if (I.isExact()) LShr->setIsExact();
- return LShr;
- }
- }
-
- if (ConstantInt *C = dyn_cast<ConstantInt>(Op1)) {
- // X udiv C, where C >= signbit
- if (C->getValue().isNegative()) {
- Value *IC = Builder->CreateICmpULT(Op0, C);
- return SelectInst::Create(IC, Constant::getNullValue(I.getType()),
- ConstantInt::get(I.getType(), 1));
- }
- }
-
// (x lshr C1) udiv C2 --> x udiv (C2 << C1)
if (ConstantInt *C2 = dyn_cast<ConstantInt>(Op1)) {
Value *X;
@@ -749,38 +799,6 @@
}
}
- // X udiv (C1 << N), where C1 is "1<<C2" --> X >> (N+C2)
- { const APInt *CI; Value *N;
- if (match(Op1, m_Shl(m_Power2(CI), m_Value(N))) ||
- match(Op1, m_ZExt(m_Shl(m_Power2(CI), m_Value(N))))) {
- if (*CI != 1)
- N = Builder->CreateAdd(N,
- ConstantInt::get(N->getType(), CI->logBase2()));
- if (ZExtInst *Z = dyn_cast<ZExtInst>(Op1))
- N = Builder->CreateZExt(N, Z->getDestTy());
- if (I.isExact())
- return BinaryOperator::CreateExactLShr(Op0, N);
- return BinaryOperator::CreateLShr(Op0, N);
- }
- }
-
- // udiv X, (Select Cond, C1, C2) --> Select Cond, (shr X, C1), (shr X, C2)
- // where C1&C2 are powers of two.
- { Value *Cond; const APInt *C1, *C2;
- if (match(Op1, m_Select(m_Value(Cond), m_Power2(C1), m_Power2(C2)))) {
- // Construct the "on true" case of the select
- Value *TSI = Builder->CreateLShr(Op0, C1->logBase2(), Op1->getName()+".t",
- I.isExact());
-
- // Construct the "on false" case of the select
- Value *FSI = Builder->CreateLShr(Op0, C2->logBase2(), Op1->getName()+".f",
- I.isExact());
-
- // construct the select instruction and return it.
- return SelectInst::Create(Cond, TSI, FSI);
- }
- }
-
// (zext A) udiv (zext B) --> zext (A udiv B)
if (ZExtInst *ZOp0 = dyn_cast<ZExtInst>(Op0))
if (Value *ZOp1 = dyn_castZExtVal(Op1, ZOp0->getSrcTy()))
@@ -788,6 +806,21 @@
I.isExact()),
I.getType());
+ // (LHS udiv (select (select (...)))) -> (LHS >> (select (select (...))))
+ SmallVector<Instruction *, 4> NewInstrs;
+ Instruction *RetI = visitUDivOperand(Op0, Op1, I, NewInstrs);
+ for (unsigned i = 0, e = NewInstrs.size(); i != e; i++)
+ // If we managed to replace the UDiv completely, insert the new intermediate
+ // instructions before where the UDiv was.
+ // If we couldn't, we must clean up after ourselves by deleting the new
+ // instructions.
+ if (RetI)
+ NewInstrs[i]->insertBefore(&I);
+ else
+ delete NewInstrs[i];
+ if (RetI)
+ return RetI;
+
return 0;
}