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_test: 

li r2, 0
LBB_test_1:     ; no_exit.2
        li r5, 0
        stw r5, 0(r3)
        addi r2, r2, 1
        addi r3, r3, 4
        cmpwi cr0, r2, 701
        blt cr0, LBB_test_1     ; no_exit.2
LBB_test_2:     ; loopexit.2.loopexit
        addi r2, r2, 1
        stw r2, 0(r4)
        blr
[zion ~/llvm]$ cat > ~/xx
Uses of IV's outside of the loop should use hte post-incremented version
of the IV, not the preincremented version.  This helps many loops (e.g. in sixtrack)
which used to generate code like this (this is the code from the
dont-hoist-simple-loop-constants.ll testcase):

_test:
        li r2, 0                 **** IV starts at 0
LBB_test_1:     ; no_exit.2
        or r5, r2, r2            **** Copy for loop exit
        li r2, 0
        stw r2, 0(r3)
        addi r3, r3, 4
        addi r2, r5, 1
        addi r6, r5, 2           **** IV+2
        cmpwi cr0, r6, 701
        blt cr0, LBB_test_1     ; no_exit.2
LBB_test_2:     ; loopexit.2.loopexit
        addi r2, r5, 2       ****  IV+2
        stw r2, 0(r4)
        blr

And now generated code like this:

_test:
        li r2, 1               *** IV starts at 1
LBB_test_1:     ; no_exit.2
        li r5, 0
        stw r5, 0(r3)
        addi r2, r2, 1
        addi r3, r3, 4
        cmpwi cr0, r2, 701     *** IV.postinc + 0
        blt cr0, LBB_test_1
LBB_test_2:     ; loopexit.2.loopexit
        stw r2, 0(r4)          *** IV.postinc + 0
        blr

llvm-svn: 23313
This commit is contained in:
Chris Lattner 2005-09-12 06:04:47 +00:00
parent 2bb00dda5a
commit a67648396a
1 changed files with 19 additions and 5 deletions
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24
llvm/lib/Transforms/Scalar/LoopStrengthReduce.cpp
View File

@ -52,7 +52,7 @@ namespace {
// isUseOfPostIncrementedValue - True if this should use the
// post-incremented version of this IV, not the preincremented version.
// This can only be set in special cases, such as the terminating setcc
// instruction for a loop.
// instruction for a loop or uses dominated by the loop.
bool isUseOfPostIncrementedValue;
IVStrideUse(const SCEVHandle &Offs, Instruction *U, Value *O)
@ -351,8 +351,17 @@ bool LoopStrengthReduce::AddUsersIfInteresting(Instruction *I, Loop *L,
if (AddUserToIVUsers) {
// Okay, we found a user that we cannot reduce. Analyze the instruction
// and decide what to do with it.
IVUsesByStride[Stride].addUser(Start, User, I);
// and decide what to do with it. If we are a use inside of the loop, use
// the value before incrementation, otherwise use it after incrementation.
if (L->contains(User->getParent())) {
IVUsesByStride[Stride].addUser(Start, User, I);
} else {
// The value used will be incremented by the stride more than we are
// expecting, so subtract this off.
SCEVHandle NewStart = SCEV::getMinusSCEV(Start, Stride);
IVUsesByStride[Stride].addUser(NewStart, User, I);
IVUsesByStride[Stride].Users.back().isUseOfPostIncrementedValue = true;
}
}
}
return true;
@ -387,7 +396,8 @@ namespace {
// isUseOfPostIncrementedValue - True if this should use the
// post-incremented version of this IV, not the preincremented version.
// This can only be set in special cases, such as the terminating setcc
// instruction for a loop.
// instruction for a loop and uses outside the loop that are dominated by
// the loop.
bool isUseOfPostIncrementedValue;
BasedUser(IVStrideUse &IVSU)
@ -842,7 +852,11 @@ void LoopStrengthReduce::StrengthReduceStridedIVUsers(const SCEVHandle &Stride,
Value *RewriteOp = NewPHI;
if (User.isUseOfPostIncrementedValue) {
RewriteOp = IncV;
User.Inst->moveBefore(LatchBlock->getTerminator());
// If this user is in the loop, make sure it is the last thing in the
// loop to ensure it is dominated by the increment.
if (L->contains(User.Inst->getParent()))
User.Inst->moveBefore(LatchBlock->getTerminator());
}
SCEVHandle RewriteExpr = SCEVUnknown::get(RewriteOp);