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Be more aggressive in doing LoadPRE by tracing backwards when a block only has

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a single predecessor.

Patch by Jakub Staszak.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@72661 91177308-0d34-0410-b5e6-96231b3b80d8
This commit is contained in:
Owen Anderson 2009-05-31 09:03:40 +00:00
parent 88b7293f49
commit 88554df4a2
2 changed files with 71 additions and 4 deletions
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43
lib/Transforms/Scalar/GVN.cpp
View File

@ -1055,11 +1055,29 @@ bool GVN::processNonLocalLoad(LoadInst *LI,
// that we only have to insert *one* load (which means we're basically moving
// the load, not inserting a new one).
// Everything we do here is based on local predecessors of LI's block. If it
// only has one predecessor, bail now.
SmallPtrSet<BasicBlock *, 4> Blockers;
for (unsigned i = 0, e = UnavailableBlocks.size(); i != e; ++i)
Blockers.insert(UnavailableBlocks[i]);
// Lets find first basic block with more than one predecessor. Walk backwards
// through predecessors if needed.
BasicBlock *LoadBB = LI->getParent();
if (LoadBB->getSinglePredecessor())
return false;
BasicBlock *TmpBB = LoadBB;
bool isSinglePred = false;
while (TmpBB->getSinglePredecessor()) {
isSinglePred = true;
TmpBB = TmpBB->getSinglePredecessor();
if (!TmpBB) // If haven't found any, bail now.
return false;
if (TmpBB == LoadBB) // Infinite (unreachable) loop.
return false;
if (Blockers.count(TmpBB))
return false;
}
assert(TmpBB);
LoadBB = TmpBB;
// If we have a repl set with LI itself in it, this means we have a loop where
// at least one of the values is LI. Since this means that we won't be able
@ -1069,6 +1087,23 @@ bool GVN::processNonLocalLoad(LoadInst *LI,
if (ValuesPerBlock[i].second == LI)
return false;
if (isSinglePred) {
bool isHot = false;
for (unsigned i = 0, e = ValuesPerBlock.size(); i != e; ++i)
if (Instruction *I = dyn_cast<Instruction>(ValuesPerBlock[i].second))
// "Hot" Instruction is in some loop (because it dominates its dep.
// instruction).
if (DT->dominates(LI, I)) {
isHot = true;
break;
}
// We are interested only in "hot" instructions. We don't want to do any
// mis-optimizations here.
if (!isHot)
return false;
}
// Okay, we have some hope :). Check to see if the loaded value is fully
// available in all but one predecessor.
// FIXME: If we could restructure the CFG, we could make a common pred with

32
test/Transforms/GVN/pre-single-pred.ll Normal file
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@ -0,0 +1,32 @@
; RUN: llvm-as < %s | opt -gvn -enable-load-pre | llvm-dis | not grep {tmp3 = load}
define i32 @f(i32* nocapture %p, i32 %n) nounwind {
entry:
br label %for.cond
for.cond: ; preds = %for.inc, %entry
%i.0 = phi i32 [ 0, %entry ], [ %indvar.next, %for.inc ] ; <i32> [#uses=2]
%cmp = icmp slt i32 %i.0, %n ; <i1> [#uses=1]
br i1 %cmp, label %for.body, label %for.cond.for.end_crit_edge
for.cond.for.end_crit_edge: ; preds = %for.cond
br label %for.end
for.body: ; preds = %for.cond
%tmp3 = load i32* %p ; <i32> [#uses=1]
%dec = add i32 %tmp3, -1 ; <i32> [#uses=2]
store i32 %dec, i32* %p
%cmp6 = icmp slt i32 %dec, 0 ; <i1> [#uses=1]
br i1 %cmp6, label %for.body.for.end_crit_edge, label %for.inc
for.body.for.end_crit_edge: ; preds = %for.body
br label %for.end
for.inc: ; preds = %for.body
%indvar.next = add i32 %i.0, 1 ; <i32> [#uses=1]
br label %for.cond
for.end: ; preds = %for.body.for.end_crit_edge, %for.cond.for.end_crit_edge
%tmp9 = load i32* %p ; <i32> [#uses=1]
ret i32 %tmp9
}