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Proper handling of diamond-like cases in if-conversion
If converter was somewhat careless about "diamond" cases, where there was no join block, or in other words, where the true/false blocks did not have analyzable branches. In such cases, it was possible for it to remove (needed) branches, resulting in a loss of entire basic blocks. Differential Revision: http://reviews.llvm.org/D16156 llvm-svn: 258310
This commit is contained in:
Krzysztof Parzyszek
parent
866bd3ac74
commit
d1195ef11e
@ -595,15 +595,19 @@ bool IfConverter::ValidDiamond(BBInfo &TrueBBI, BBInfo &FalseBBI,
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// Now, in preparation for counting duplicate instructions at the ends of the
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// blocks, move the end iterators up past any branch instructions.
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while (TIE != TIB) {
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--TIE;
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if (!TIE->isBranch())
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break;
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}
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while (FIE != FIB) {
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--FIE;
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if (!FIE->isBranch())
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break;
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// If both blocks are returning don't skip the branches, since they will
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// likely be both identical return instructions. In such cases the return
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// can be left unpredicated.
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// Check for already containing all of the block.
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if (TIB == TIE || FIB == FIE)
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return true;
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--TIE;
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--FIE;
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if (!TrueBBI.BB->succ_empty() || !FalseBBI.BB->succ_empty()) {
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while (TIE != TIB && TIE->isBranch())
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--TIE;
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while (FIE != FIB && FIE->isBranch())
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--FIE;
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}
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// If Dups1 includes all of a block, then don't count duplicate
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@ -1395,8 +1399,13 @@ bool IfConverter::IfConvertDiamond(BBInfo &BBI, IfcvtKind Kind,
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BBI.BB->splice(BBI.BB->end(), BBI1->BB, BBI1->BB->begin(), DI1);
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BBI2->BB->erase(BBI2->BB->begin(), DI2);
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// Remove branch from 'true' block and remove duplicated instructions.
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BBI1->NonPredSize -= TII->RemoveBranch(*BBI1->BB);
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// Remove branch from the 'true' block, unless it was not analyzable.
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// Non-analyzable branches need to be preserved, since in such cases,
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// the CFG structure is not an actual diamond (the join block may not
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// be present).
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if (BBI1->IsBrAnalyzable)
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BBI1->NonPredSize -= TII->RemoveBranch(*BBI1->BB);
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// Remove duplicated instructions.
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DI1 = BBI1->BB->end();
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for (unsigned i = 0; i != NumDups2; ) {
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// NumDups2 only counted non-dbg_value instructions, so this won't
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@ -1413,8 +1422,10 @@ bool IfConverter::IfConvertDiamond(BBInfo &BBI, IfcvtKind Kind,
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// must be removed.
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RemoveKills(BBI1->BB->begin(), BBI1->BB->end(), DontKill, *TRI);
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// Remove 'false' block branch and find the last instruction to predicate.
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BBI2->NonPredSize -= TII->RemoveBranch(*BBI2->BB);
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// Remove 'false' block branch (unless it was not analyzable), and find
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// the last instruction to predicate.
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if (BBI2->IsBrAnalyzable)
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BBI2->NonPredSize -= TII->RemoveBranch(*BBI2->BB);
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DI2 = BBI2->BB->end();
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while (NumDups2 != 0) {
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// NumDups2 only counted non-dbg_value instructions, so this won't
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@ -1473,6 +1484,18 @@ bool IfConverter::IfConvertDiamond(BBInfo &BBI, IfcvtKind Kind,
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// Predicate the 'true' block.
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PredicateBlock(*BBI1, BBI1->BB->end(), *Cond1, &RedefsByFalse);
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// After predicating BBI1, if there is a predicated terminator in BBI1 and
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// a non-predicated in BBI2, then we don't want to predicate the one from
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// BBI2. The reason is that if we merged these blocks, we would end up with
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// two predicated terminators in the same block.
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if (!BBI2->BB->empty() && (DI2 == BBI2->BB->end())) {
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MachineBasicBlock::iterator BBI1T = BBI1->BB->getFirstTerminator();
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MachineBasicBlock::iterator BBI2T = BBI2->BB->getFirstTerminator();
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if ((BBI1T != BBI1->BB->end()) && TII->isPredicated(BBI1T) &&
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((BBI2T != BBI2->BB->end()) && !TII->isPredicated(BBI2T)))
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--DI2;
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}
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// Predicate the 'false' block.
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PredicateBlock(*BBI2, DI2, *Cond2);
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@ -1488,6 +1511,12 @@ bool IfConverter::IfConvertDiamond(BBInfo &BBI, IfcvtKind Kind,
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BBInfo &TailBBI = BBAnalysis[TailBB->getNumber()];
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bool CanMergeTail = !TailBBI.HasFallThrough &&
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!TailBBI.BB->hasAddressTaken();
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// The if-converted block can still have a predicated terminator
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// (e.g. a predicated return). If that is the case, we cannot merge
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// it with the tail block.
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MachineBasicBlock::const_iterator TI = BBI.BB->getFirstTerminator();
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if (TI != BBI.BB->end() && TII->isPredicated(TI))
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CanMergeTail = false;
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// There may still be a fall-through edge from BBI1 or BBI2 to TailBB;
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// check if there are any other predecessors besides those.
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unsigned NumPreds = TailBB->pred_size();
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@ -1659,8 +1688,16 @@ void IfConverter::MergeBlocks(BBInfo &ToBBI, BBInfo &FromBBI, bool AddEdges) {
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assert(!FromBBI.BB->hasAddressTaken() &&
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"Removing a BB whose address is taken!");
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ToBBI.BB->splice(ToBBI.BB->end(),
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FromBBI.BB, FromBBI.BB->begin(), FromBBI.BB->end());
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// In case FromBBI.BB contains terminators (e.g. return instruction),
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// first move the non-terminator instructions, then the terminators.
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MachineBasicBlock::iterator FromTI = FromBBI.BB->getFirstTerminator();
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MachineBasicBlock::iterator ToTI = ToBBI.BB->getFirstTerminator();
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ToBBI.BB->splice(ToTI, FromBBI.BB, FromBBI.BB->begin(), FromTI);
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// If FromBB has non-predicated terminator we should copy it at the end.
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if ((FromTI != FromBBI.BB->end()) && !TII->isPredicated(FromTI))
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ToTI = ToBBI.BB->end();
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ToBBI.BB->splice(ToTI, FromBBI.BB, FromTI, FromBBI.BB->end());
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// Force normalizing the successors' probabilities of ToBBI.BB to convert all
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// unknown probabilities into known ones.
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43
test/CodeGen/Hexagon/ifcvt-diamond-bad.ll
Normal file
43
test/CodeGen/Hexagon/ifcvt-diamond-bad.ll
Normal file
@ -0,0 +1,43 @@
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; RUN: llc -march=hexagon -minimum-jump-tables=1 < %s
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; REQUIRES: asserts
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target datalayout = "e-m:e-p:32:32:32-i64:64:64-i32:32:32-i16:16:16-i1:8:8-f64:64:64-f32:32:32-v64:64:64-v32:32:32-a:0-n16:32"
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target triple = "hexagon"
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%struct.t0 = type { i8, [2 x i8] }
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%struct.t1 = type { i8, i8, [1900 x i8], %struct.t0 }
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@var = internal global [3 x %struct.t1] zeroinitializer, align 8
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declare void @foo() #2
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declare void @bar(i32, i32) #2
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; Function Attrs: nounwind
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define void @fred(i8 signext %a, i8 signext %b) #1 {
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entry:
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%i = sext i8 %a to i32
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%t = getelementptr inbounds [3 x %struct.t1], [3 x %struct.t1]* @var, i32 0, i32 %i, i32 3, i32 0
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%0 = load i8, i8* %t, align 8
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switch i8 %0, label %if.end14 [
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i8 1, label %if.then
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i8 0, label %do.body
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]
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if.then: ; preds = %entry
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%j = sext i8 %b to i32
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%u = getelementptr inbounds [3 x %struct.t1], [3 x %struct.t1]* @var, i32 0, i32 %i, i32 3, i32 1, i32 %j
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store i8 1, i8* %u, align 1
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tail call void @foo() #0
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br label %if.end14
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do.body: ; preds = %entry
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%conv11 = sext i8 %b to i32
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tail call void @bar(i32 %i, i32 %conv11) #0
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br label %if.end14
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if.end14: ; preds = %entry, %do.body, %if.then
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ret void
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}
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attributes #0 = { nounwind }
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attributes #1 = { nounwind "disable-tail-calls"="false" }
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attributes #2 = { "disable-tail-calls"="false" }
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