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[LICM] Don't create more than one copy of an instruction per loop exit block when sinking.

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Fixes exponential compilation complexity in PR19835, caused by
LICM::sink not handling the following pattern well:

f = op g
e = op f, g
d = op e
c = op d, e
b = op c
a = op b, c

When an instruction with N uses is sunk, each of its operands gets N
new uses (all of them - phi nodes). In the example above, if a had 1
use, c would have 2, e would have 4, and g would have 8.

llvm-svn: 211673
This commit is contained in:
Evgeniy Stepanov 2014-06-25 07:54:58 +00:00
parent bbdd74d0fb
commit aeb724c213
2 changed files with 62 additions and 23 deletions
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56
lib/Transforms/Scalar/LICM.cpp
View File

@ -550,6 +550,9 @@ void LICM::sink(Instruction &I) {
SmallPtrSet<BasicBlock *, 32> ExitBlockSet(ExitBlocks.begin(), ExitBlocks.end());
#endif
// Clones of this instruction. Don't create more than one per exit block!
SmallDenseMap<BasicBlock *, Instruction *, 32> SunkCopies;
// If this instruction is only used outside of the loop, then all users are
// PHI nodes in exit blocks due to LCSSA form. Just RAUW them with clones of
// the instruction.
@ -561,30 +564,37 @@ void LICM::sink(Instruction &I) {
assert(ExitBlockSet.count(ExitBlock) &&
"The LCSSA PHI is not in an exit block!");
Instruction *New = I.clone();
ExitBlock->getInstList().insert(ExitBlock->getFirstInsertionPt(), New);
if (!I.getName().empty())
New->setName(I.getName() + ".le");
Instruction *New;
auto It = SunkCopies.find(ExitBlock);
if (It != SunkCopies.end()) {
New = It->second;
} else {
New = I.clone();
SunkCopies[ExitBlock] = New;
ExitBlock->getInstList().insert(ExitBlock->getFirstInsertionPt(), New);
if (!I.getName().empty())
New->setName(I.getName() + ".le");
// Build LCSSA PHI nodes for any in-loop operands. Note that this is
// particularly cheap because we can rip off the PHI node that we're
// replacing for the number and blocks of the predecessors.
// OPT: If this shows up in a profile, we can instead finish sinking all
// invariant instructions, and then walk their operands to re-establish
// LCSSA. That will eliminate creating PHI nodes just to nuke them when
// sinking bottom-up.
for (User::op_iterator OI = New->op_begin(), OE = New->op_end(); OI != OE;
++OI)
if (Instruction *OInst = dyn_cast<Instruction>(*OI))
if (Loop *OLoop = LI->getLoopFor(OInst->getParent()))
if (!OLoop->contains(PN)) {
PHINode *OpPN = PHINode::Create(
OInst->getType(), PN->getNumIncomingValues(),
OInst->getName() + ".lcssa", ExitBlock->begin());
for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i)
OpPN->addIncoming(OInst, PN->getIncomingBlock(i));
*OI = OpPN;
}
// Build LCSSA PHI nodes for any in-loop operands. Note that this is
// particularly cheap because we can rip off the PHI node that we're
// replacing for the number and blocks of the predecessors.
// OPT: If this shows up in a profile, we can instead finish sinking all
// invariant instructions, and then walk their operands to re-establish
// LCSSA. That will eliminate creating PHI nodes just to nuke them when
// sinking bottom-up.
for (User::op_iterator OI = New->op_begin(), OE = New->op_end(); OI != OE;
++OI)
if (Instruction *OInst = dyn_cast<Instruction>(*OI))
if (Loop *OLoop = LI->getLoopFor(OInst->getParent()))
if (!OLoop->contains(PN)) {
PHINode *OpPN = PHINode::Create(
OInst->getType(), PN->getNumIncomingValues(),
OInst->getName() + ".lcssa", ExitBlock->begin());
for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i)
OpPN->addIncoming(OInst, PN->getIncomingBlock(i));
*OI = OpPN;
}
}
PN->replaceAllUsesWith(New);
PN->eraseFromParent();

29
test/Transforms/LICM/extra-copies.ll Normal file
View File

@ -0,0 +1,29 @@
; RUN: opt < %s -licm -S | FileCheck %s
; PR19835
target datalayout = "e-m:e-i64:64-f80:128-n8:16:32:64-S128"
target triple = "x86_64-unknown-linux-gnu"
define i32 @f(i32 %x) {
entry:
br label %for.body
for.body: ; preds = %entry, %for.body
%storemerge4 = phi i32 [ 0, %entry ], [ %inc, %for.body ]
%mul = mul nsw i32 %x, %x
%add2 = add nsw i32 %mul, %x
%mul3 = add nsw i32 %add2, %mul
%inc = add nsw i32 %storemerge4, 1
%cmp = icmp slt i32 %inc, 100
br i1 %cmp, label %for.body, label %for.end
for.end: ; preds = %for.body
%a9.0.lcssa = phi i32 [ %mul3, %for.body ]
ret i32 %a9.0.lcssa
}
; Test that there is exactly one copy of mul nsw i32 %x, %x in the exit block.
; CHECK: define i32 @f(i32 [[X:%.*]])
; CHECK: for.end:
; CHECK-NOT: mul nsw i32 [[X]], [[X]]
; CHECK: mul nsw i32 [[X]], [[X]]
; CHECK-NOT: mul nsw i32 [[X]], [[X]]