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Speculatively execute a block when the the block is the then part of a triangle shape and it contains a single, side effect free, cheap instruction. The branch is eliminated by adding a select instruction. i.e.

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Turn                                                                                                                                                                                                       
BB:                                                                                                                                                                                                        
    %t1 = icmp                                                                                                                                                                                             
    br i1 %t1, label %BB1, label %BB2                                                                                                                                                                      
BB1:                                                                                                                                                                                                       
    %t3 = add %t2, c                                                                                                                                                                                       
    br label BB2                                                                                                                                                                                           
BB2:                                                                                                                                                                                                       
=>                                                                                                                                                                                                         
BB:                                                                                                                                                                                                        
    %t1 = icmp                                                                                                                                                                                             
    %t4 = add %t2, c                                                                                                                                                                                       
    %t3 = select i1 %t1, %t2, %t3


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@52073 91177308-0d34-0410-b5e6-96231b3b80d8
This commit is contained in:
Evan Cheng 2008-06-07 08:52:29 +00:00
parent cce302fded
commit 4d09efd7b8
2 changed files with 142 additions and 0 deletions
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121
lib/Transforms/Utils/SimplifyCFG.cpp
View File

@ -955,6 +955,109 @@ HoistTerminator:
return true;
}
/// SpeculativelyExecuteBB - Given a conditional branch that goes to BB1
/// and an BB2 and the only successor of BB1 is BB2, hoist simple code
/// (for now, restricted to a single instruction that's side effect free) from
/// the BB1 into the branch block to speculatively execute it.
static bool SpeculativelyExecuteBB(BranchInst *BI, BasicBlock *BB1) {
// Only speculatively execution a single instruction (not counting the
// terminator) for now.
if (BB1->size() != 2)
return false;
// If BB1 is actually on the false edge of the conditional branch, remember
// to swap the select operands later.
bool Invert = false;
if (BB1 != BI->getSuccessor(0)) {
assert(BB1 == BI->getSuccessor(1) && "No edge from 'if' block?");
Invert = true;
}
// Turn
// BB:
// %t1 = icmp
// br i1 %t1, label %BB1, label %BB2
// BB1:
// %t3 = add %t2, c
// br label BB2
// BB2:
// =>
// BB:
// %t1 = icmp
// %t4 = add %t2, c
// %t3 = select i1 %t1, %t2, %t3
Instruction *I = BB1->begin();
switch (I->getOpcode()) {
default: return false; // Not safe / profitable to hoist.
case Instruction::Add:
case Instruction::Sub:
case Instruction::And:
case Instruction::Or:
case Instruction::Xor:
case Instruction::Shl:
case Instruction::LShr:
case Instruction::AShr:
if (I->getOperand(0)->getType()->isFPOrFPVector())
return false; // FP arithmetic might trap.
break; // These are all cheap and non-trapping instructions.
}
// Can we speculatively execute the instruction? And what is the value
// if the condition is false? Consider the phi uses, if the incoming value
// from the "if" block are all the same V, then V is the value of the
// select if the condition is false.
BasicBlock *BIParent = BI->getParent();
SmallVector<PHINode*, 4> PHIUses;
Value *FalseV = NULL;
for (Value::use_iterator UI = I->use_begin(), E = I->use_end();
UI != E; ++UI) {
PHINode *PN = dyn_cast<PHINode>(UI);
if (!PN)
continue;
PHIUses.push_back(PN);
Value *PHIV = PN->getIncomingValueForBlock(BIParent);
if (!FalseV)
FalseV = PHIV;
else if (FalseV != PHIV)
return false; // Don't know the value when condition is false.
}
if (!FalseV) // Can this happen?
return false;
// If we get here, we can hoist the instruction. Try to place it before the
// icmp / fcmp instruction preceeding the conditional branch.
BasicBlock::iterator InsertPos = BI;
if (InsertPos != BIParent->begin())
--InsertPos;
if (InsertPos->getOpcode() == Instruction::ICmp ||
InsertPos->getOpcode() == Instruction::FCmp)
BIParent->getInstList().splice(InsertPos, BB1->getInstList(), I);
else
BIParent->getInstList().splice(BI, BB1->getInstList(), I);
// Create a select whose true value is the speculatively executed value and
// false value is the previously determined FalseV.
SelectInst *SI;
if (Invert)
SI = SelectInst::Create(BI->getCondition(), FalseV, I,
FalseV->getName() + "." + I->getName(), BI);
else
SI = SelectInst::Create(BI->getCondition(), I, FalseV,
I->getName() + "." + FalseV->getName(), BI);
// Make the PHI node use the select for all incoming values for "then" and
// "if" blocks.
for (unsigned i = 0, e = PHIUses.size(); i != e; ++i) {
PHINode *PN = PHIUses[i];
for (unsigned j = 0, ee = PN->getNumIncomingValues(); j != ee; ++j)
if (PN->getIncomingBlock(j) == BB1 ||
PN->getIncomingBlock(j) == BIParent)
PN->setIncomingValue(j, SI);
}
return true;
}
/// BlockIsSimpleEnoughToThreadThrough - Return true if we can thread a branch
/// across this block.
static bool BlockIsSimpleEnoughToThreadThrough(BasicBlock *BB) {
@ -1928,6 +2031,24 @@ bool llvm::SimplifyCFG(BasicBlock *BB) {
// so see if there is any identical code in the "then" and "else"
// blocks. If so, we can hoist it up to the branching block.
Changed |= HoistThenElseCodeToIf(BI);
} else {
OnlySucc = NULL;
for (succ_iterator SI = succ_begin(BB), SE = succ_end(BB);
SI != SE; ++SI) {
if (!OnlySucc)
OnlySucc = *SI;
else if (*SI != OnlySucc) {
OnlySucc = 0; // There are multiple distinct successors!
break;
}
}
if (OnlySucc == OtherBB) {
// If BB's only successor is the other successor of the predecessor,
// i.e. a triangle, see if we can hoist any code from this block up
// to the "if" block.
Changed |= SpeculativelyExecuteBB(BI, BB);
}
}
}

21
test/Transforms/SimplifyCFG/SpeculativeExec.ll Normal file
View File

@ -0,0 +1,21 @@
; RUN: llvm-as < %s | opt -simplifycfg | llvm-dis | grep select
; RUN: llvm-as < %s | opt -simplifycfg | llvm-dis | grep br | count 2
define i32 @t2(i32 %a, i32 %b, i32 %c) nounwind {
entry:
%tmp1 = icmp eq i32 %b, 0
br i1 %tmp1, label %bb1, label %bb3
bb1: ; preds = %entry
%tmp2 = icmp sgt i32 %c, 1
br i1 %tmp2, label %bb2, label %bb3
bb2: ; preds = bb1
%tmp3 = add i32 %a, 1
br label %bb3
bb3: ; preds = %bb2, %entry
%tmp4 = phi i32 [ %b, %entry ], [ %a, %bb1 ], [ %tmp3, %bb2 ]
%tmp5 = sub i32 %tmp4, 1
ret i32 %tmp5
}