[LLVMdev] Remove identical or redundant basic blocks?

[Heinz Riener]

Dale is totally right, all of these blocks disappear in later
target-dependent optimizations.  I have not thought about that since
eliminating these blocks requires no target-dependent information.
However, I guess it is not worth eliminating them earlier.

John, I tried your advice and executed opt (after -O3) again with
-mergereturn and -simplifycfg: The -mergereturn pass introduces
another basic block, called UnifiedReturnBlock, and replaces ret
instructions with jumps to UnifiedReturnBlock.  So, it does not
eliminates any block but it creates one. (I think its task is to merge
the ret instructions (and not the blocks).)

The -simplifycfg pass does not affect my code at all.  Moreover,
according to 'StandardPasses.h' I guess the pass already runs (about 4
times) as a part of -O3.  I have yet not looked at the actual code of
simplifycfg but the comment in 'StandarPasses.h' says it should merge
and eliminate basic blocks.

include/llvm/Support/StandardPasses.h, line 129 (SVN-97366):
PM->add(createCFGSimplificationPass());     // Merge & remove BBs

Heinz

On 05/09/2010 01:36 PM, John Criswell wrote:
> Eugene Toder wrote:
>> Would it make sense to have a similar pass that operates on llvm main IR?
>>
>
> The unify exit return node (-mergereturn?) pass should take care of the
> first example.  The -simplifycfg option might take care of the second
> example.  Both work on LLVM IR.
>
> -- John T.
>> On Sat, May 8, 2010 at 5:15 PM, Dale Johannesen<dalej at apple.com>  wrote:
>>
>>> The branch folding pass does this, but it operates later, on the
>>> target-dependent form in llc.
>>>
>>> On May 8, 2010, at 8:48 AM, Heinz Riener wrote:
>>>
>>>
>>>> Dear all,
>>>>
>>>> after optimizing a small LLVM example program (i.e., with opt -O3),
>>>> the
>>>> resulting code contains several basic blocks, which I consider
>>>> identical
>>>> or redundant.  For instance,
>>>>
>>>> return:                                           ; preds = %min.exit
>>>>    ret i32 0
>>>>
>>>> bb15:                                             ; preds = %min.exit
>>>>    ret i32 0
>>>>
>>>> or,
>>>>
>>>> bb7.i:                                            ; preds = %bb1.i
>>>>    br label %bb3.i
>>>>
>>>> bb9.i:                                            ; preds = %bb1.i
>>>>    br label %bb3.i
>>>>
>>>> I think it is safe to remove the blocks bb7.i, bb9.i, bb15 while
>>>> replacing each jump to bb15 with return and jumps to bb7.i, bb9.i with
>>>> bb3.i.
>>>>
>>>> Is there any opt pass I can apply to the code to merge or remove these
>>>> blocks?
>>>>
>>>> Heinz
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>>
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