[InstCombine] fold zexts and constants into a phi (PR24766)
This is one step towards solving PR24766:
https://llvm.org/bugs/show_bug.cgi?id=24766
We were not producing the same IR for these two C functions because the store
to the temp bool causes extra zexts:
#include <stdbool.h>
bool switchy(char x1, char x2, char condition) {
bool conditionMet = false;
switch (condition) {
case 0: conditionMet = (x1 == x2); break;
case 1: conditionMet = (x1 <= x2); break;
}
return conditionMet;
}
bool switchy2(char x1, char x2, char condition) {
switch (condition) {
case 0: return (x1 == x2);
case 1: return (x1 <= x2);
}
return false;
}
As noted in the code comments, this test case manages to avoid the more general existing
phi optimizations where there are only 2 phi inputs or where there are no constant phi
args mixed in with the casts ops. It seems like a corner case, but if we don't catch it,
then I don't think we can get SimplifyCFG to further optimize towards the canonical form
for this function shown in the bug report.
Differential Revision: http://reviews.llvm.org/D12866
llvm-svn: 248689
diff --git a/llvm/lib/Transforms/InstCombine/InstCombinePHI.cpp b/llvm/lib/Transforms/InstCombine/InstCombinePHI.cpp
index 86d5f03..b035dd6 100644
--- a/llvm/lib/Transforms/InstCombine/InstCombinePHI.cpp
+++ b/llvm/lib/Transforms/InstCombine/InstCombinePHI.cpp
@@ -394,7 +394,73 @@
return NewLI;
}
+/// TODO: This function could handle other cast types, but then it might
+/// require special-casing a cast from the 'i1' type. See the comment in
+/// FoldPHIArgOpIntoPHI() about pessimizing illegal integer types.
+Instruction *InstCombiner::FoldPHIArgZextsIntoPHI(PHINode &Phi) {
+ // Early exit for the common case of a phi with two operands. These are
+ // handled elsewhere. See the comment below where we check the count of zexts
+ // and constants for more details.
+ unsigned NumIncomingValues = Phi.getNumIncomingValues();
+ if (NumIncomingValues < 3)
+ return nullptr;
+ // Find the narrower type specified by the first zext.
+ Type *NarrowType = nullptr;
+ for (Value *V : Phi.incoming_values()) {
+ if (auto *Zext = dyn_cast<ZExtInst>(V)) {
+ NarrowType = Zext->getSrcTy();
+ break;
+ }
+ }
+ if (!NarrowType)
+ return nullptr;
+
+ // Walk the phi operands checking that we only have zexts or constants that
+ // we can shrink for free. Store the new operands for the new phi.
+ SmallVector<Value *, 4> NewIncoming;
+ unsigned NumZexts = 0;
+ unsigned NumConsts = 0;
+ for (Value *V : Phi.incoming_values()) {
+ if (auto *Zext = dyn_cast<ZExtInst>(V)) {
+ // All zexts must be identical and have one use.
+ if (Zext->getSrcTy() != NarrowType || !Zext->hasOneUse())
+ return nullptr;
+ NewIncoming.push_back(Zext->getOperand(0));
+ NumZexts++;
+ } else if (auto *C = dyn_cast<Constant>(V)) {
+ // Make sure that constants can fit in the new type.
+ Constant *Trunc = ConstantExpr::getTrunc(C, NarrowType);
+ if (ConstantExpr::getZExt(Trunc, C->getType()) != C)
+ return nullptr;
+ NewIncoming.push_back(Trunc);
+ NumConsts++;
+ } else {
+ // If it's not a cast or a constant, bail out.
+ return nullptr;
+ }
+ }
+
+ // The more common cases of a phi with no constant operands or just one
+ // variable operand are handled by FoldPHIArgOpIntoPHI() and FoldOpIntoPhi()
+ // respectively. FoldOpIntoPhi() wants to do the opposite transform that is
+ // performed here. It tries to replicate a cast in the phi operand's basic
+ // block to expose other folding opportunities. Thus, InstCombine will
+ // infinite loop without this check.
+ if (NumConsts == 0 || NumZexts < 2)
+ return nullptr;
+
+ // All incoming values are zexts or constants that are safe to truncate.
+ // Create a new phi node of the narrow type, phi together all of the new
+ // operands, and zext the result back to the original type.
+ PHINode *NewPhi = PHINode::Create(NarrowType, NumIncomingValues,
+ Phi.getName() + ".shrunk");
+ for (unsigned i = 0; i != NumIncomingValues; ++i)
+ NewPhi->addIncoming(NewIncoming[i], Phi.getIncomingBlock(i));
+
+ InsertNewInstBefore(NewPhi, Phi);
+ return CastInst::CreateZExtOrBitCast(NewPhi, Phi.getType());
+}
/// If all operands to a PHI node are the same "unary" operator and they all are
/// only used by the PHI, PHI together their inputs, and do the operation once,
@@ -800,6 +866,9 @@
if (Value *V = SimplifyInstruction(&PN, DL, TLI, DT, AC))
return ReplaceInstUsesWith(PN, V);
+ if (Instruction *Result = FoldPHIArgZextsIntoPHI(PN))
+ return Result;
+
// If all PHI operands are the same operation, pull them through the PHI,
// reducing code size.
if (isa<Instruction>(PN.getIncomingValue(0)) &&