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Throttle back tail duplication to avoid creating really ugly sequences of code.

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For Transforms/TailDup/if-tail-dup.ll, f.e., it produces:

_foo:
        movl 8(%esp), %eax
        movl 4(%esp), %ecx
        testl $1, %ecx
        je LBB1_2       #cond_next
LBB1_1: #cond_true
        movl $1, (%eax)
LBB1_2: #cond_next
        testl $2, %ecx
        je LBB1_4       #cond_next10
LBB1_3: #cond_true6
        movl $1, 4(%eax)
LBB1_4: #cond_next10
        testl $4, %ecx
        je LBB1_6       #cond_next18
LBB1_5: #cond_true14
        movl $1, 8(%eax)
LBB1_6: #cond_next18
        testl $8, %ecx
        je LBB1_8       #return
LBB1_7: #cond_true22
        movl $1, 12(%eax)
        ret
LBB1_8: #return
        ret

instead of:

_foo:
        movl 4(%esp), %eax
        testl $2, %eax
        sete %cl
        movl 8(%esp), %edx
        testl $1, %eax
        je LBB1_2       #cond_next
LBB1_1: #cond_true
        movl $1, (%edx)
        testb %cl, %cl
        jne LBB1_4      #cond_next10
        jmp LBB1_3      #cond_true6
LBB1_2: #cond_next
        testb %cl, %cl
        jne LBB1_4      #cond_next10
LBB1_3: #cond_true6
        movl $1, 4(%edx)
        testl $4, %eax
        je LBB1_6       #cond_next18
        jmp LBB1_5      #cond_true14
LBB1_4: #cond_next10
        testl $4, %eax
        je LBB1_6       #cond_next18
LBB1_5: #cond_true14
        movl $1, 8(%edx)
        testl $8, %eax
        je LBB1_8       #return
        jmp LBB1_7      #cond_true22
LBB1_6: #cond_next18
        testl $8, %eax
        je LBB1_8       #return
LBB1_7: #cond_true22
        movl $1, 12(%edx)
        ret
LBB1_8: #return
        ret


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@30158 91177308-0d34-0410-b5e6-96231b3b80d8
This commit is contained in:
Chris Lattner 2006-09-07 21:30:15 +00:00
parent 3d222b9ace
commit e99c623e75
1 changed files with 19 additions and 0 deletions
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19
lib/Transforms/Scalar/TailDuplication.cpp
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@ -127,6 +127,25 @@ bool TailDup::shouldEliminateUnconditionalBranch(TerminatorInst *TI) {
for (; PI != PE; ++PI)
if (TooMany-- == 0) return false;
}
// Finally, if this unconditional branch is a fall-through, be careful about
// tail duplicating it. In particular, we don't want to taildup it if the
// original block will still be there after taildup is completed: doing so
// would eliminate the fall-through, requiring unconditional branches.
Function::iterator DestI = Dest;
if (&*--DestI == BI->getParent()) {
// The uncond branch is a fall-through. Tail duplication of the block is
// will eliminate the fall-through-ness and end up cloning the terminator
// at the end of the Dest block. Since the original Dest block will
// continue to exist, this means that one or the other will not be able to
// fall through. One typical example that this helps with is code like:
// if (a)
// foo();
// if (b)
// foo();
// Cloning the 'if b' block into the end of the first foo block is messy.
return false;
}
return true;
}