[LLVMdev] What are the optimizations that gold uses during the final link stage?

Devang Patel dpatel at apple.com
Tue Aug 31 10:38:26 PDT 2010

On Aug 31, 2010, at 9:54 AM, Rafael Espindola wrote:

> On 19 August 2010 01:25, Guoliang Jin <jingl1345 at gmail.com> wrote:
>>  Hi,
>> I am using gold plugin with the option "-use-gold-plugin
>> -Wl,-plugin-opt=also-emit-llvm" to compile apache.
>> I objdumped the final executable httpd to httpd.S.
>> I also compiled those per file bytecode into native code using the
>> following:
>> llc filename.o
>> llvm-gcc -c filename.o.s -o filename.o.o
>> Then I objdumped filename.o.o to filename.o.S and compared some
>> functions between httpd.S. and filename.o.S, and I found the order of
>> some instructions are different.
>> Is this the expected behavior, probably because of some optimization
>> during the final link stage? If this is the case, can you point me what
>> those optimizations are, so that I could also apply them while compiling
>> per file bytecode to native code.
>> Or am I using the wrong way to compile per file bytecode to native code?
> Gold uses the libLTO interface. I think there is a small bug (strange
> design?) in libLTO: lto_codegen_write_merged_modules will write the
> bitcode files after they are merged and internalized, but before other
> optimizations are run. We should probably change that or add a new
> function to libLTO that gives us access to the optimized code.
> Can someone more familiar with libLTO comment why write_merged_modules
> writes the IL before it is optimized?

I am not sure why "before it is optimized?". May be this is gold specific ?

One of the use of this utility is a debugging aid. The linker can select to do this when -save-temps is used on the command line.


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