[LLVMdev] Does the gcc frontend do inlining or deadcode elimination ?

[Long Fei]

This didn't work as I tried with 197.parser. it works without 
"-Wl,-disable-opt" switch though.

[197.parser]$ llvm-gcc analyze-linkage.c and.c build-disjuncts.c 
extract-links.c fast-match.c idiom.c main.c massage.c parse.c 
post-process.c print.c prune.c read-dict.c utilities.c xalloc.c 
word-file.c strncasecmp.c -Wa,-disable-opt -Wl,-disable-opt -lm -o 
llvm_parser
[197.parser]$ opt -inline -inline-threshold=200 < llvm_parser.bc | llc 
-march=c > parser_inline.c                       
llc: bytecode didn't read correctly.

Does opt call Transform/IPO/InlineSimple to do inlining ? as I added 
instrumentation into it, I found:
1) getInlineCost() is called when doing llvm-gcc (without the -disable* 
flags)
2) getInlineCost() is not called when doing opt

does the .bc code emitted by llvm-gcc carry inlining cost info ? 
otherwise, how does opt do inlining without the cost info ?

thanks,
--Long

Misha Brukman wrote:

On Thu, Jul 07, 2005 at 03:52:42PM -0500, Long Fei wrote:

>I noticed that the inlining condition (in
>Transforms/IPO/InlineSimple.cpp) is tested during llvm-gcc but not
>during the opt phase ? Can anybody explain what happens during
>llvm-gcc and opt respectively ?
>  
>

John answered the llvm-gcc part, so let me address the opt part.

`opt' is a modular optimizer, but it will do exactly what you tell it to
do, and nothing more, so if you say "opt -inline < input.bc > output.bc"
it will only inline.  Note that if you say "opt < old.bc > new.bc" opt
will do nothing.

This differs from gccas and gccld which have a built-in list of
optimizations that they run, which you can get a list of if you follow
the directions here:

http://llvm.cs.uiuc.edu/docs/HowToSubmitABug.html#gccas
http://llvm.cs.uiuc.edu/docs/HowToSubmitABug.html#gccld

or just read their source code.


John Criswell wrote:

> Long Fei wrote:
>
>>
>> I am investigating some inlining issue, so I did
>>
>> llvm-gcc aaa.c bbb.c ... nnn.c -o output
>> opt -inline -inline-threshold=xxx < output.bc | llc -march=c > 
>> output_inline.c
>
>
> I am unsure of whether the LLVM GCC frontend does any inlining. 
> However, I do know that your methods above run the LLVM inlining pass, 
> albeit indirectly.
>
> If you use llvm-gcc to generate and link LLVM bytecode files (as you 
> do in the example above), llvm-gcc runs gccas and gccld (the 
> optimizing assembler and optimizing linker, respectively).  Both gccas 
> and gccld run various LLVM optimizations, including inlining.  This 
> allows llvm-gcc to automatically perform interprocedural optimizations.
>
> To get a completely unoptimized bytecode file, do the following:
>
> llvm-gcc aaa.c bbb.c ... nnn.c -Wa,-disable-opt -Wl,-disable-opt -o 
> output
>
> That should disable all LLVM optimizations.
>
> -- John T.
>
>>
>> 1)
>> I noticed that even if I set xxx to 0 or even a very small negative 
>> number, many functions are eliminated. I am wondering if these 
>> functions are inlined by the frontend, or identified as deadcode.
>>
>> For instance, SPEC2k bzip2 has 79 functions, after these two steps, 
>> only 61 functions are left. no other optimizations are used.
>>
>> 2)
>> I noticed that the inlining condition (in 
>> Transforms/IPO/InlineSimple.cpp) is tested during llvm-gcc but not 
>> during the opt phase ? Can anybody explain what happens during 
>> llvm-gcc and opt respectively ?
>>
>>
>> thanks,
>> --Long
>>
>>
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>
>
>