[llvm-dev] How to get generic datalayout and "target-cpu" attribute

[Manuel Jacob via llvm-dev]

Hi Mehdi,

On 2016-01-15 02:52, Mehdi Amini wrote:
> It seems that you may want to have a look at
> http://llvm.org/docs/doxygen/html/Host_8h_source.html

I think this will give me the "fastest" CPU model and feature set 
available.  However, I want a generic model, which is "x86-64" in my 
case.

> Also, your compiler could easily just produce the final object file
> directly instead of calling opt/llc (mimic opt.cpp and llc.cpp for
> instance, or what clang is doing).

Sorry for not having stated that more explicitly.  My compiler can only 
output a text file and doesn't link against LLVM libraries.

> —
> Mehdi
> 
>> On Jan 14, 2016, at 5:31 PM, Manuel Jacob via llvm-dev 
>> <llvm-dev at lists.llvm.org> wrote:
>> 
>> Hi,
>> 
>> My compiler emits LLVM IR in text form and passes it to the opt and 
>> llc executables.
>> 
>> The datalayout and triple are currently determined by running "clang 
>> -x c /dev/null -emit-llvm -S -o -" and parsing the output.  This works 
>> fine because clang seems to choose a generic version of the CPU it is 
>> running on, and this is exactly what we want.  Still this solution is 
>> hacky and I'd like to know whether there is a cleaner way to achieve 
>> this.
>> 
>> The "target-cpu" attribute is not set at all by my compiler.  I'm 
>> concerned that this limits the effectiveness of certain optimization 
>> passes.  I don't understand when the triple is used and when the 
>> "target-cpu" attribute is used.
>> 
>> The compiler links some other IR, which is generated by clang, into 
>> the main IR output of the compiler before optimization.  The functions 
>> pulled in from the clang outputs have the "target-cpu" attribute set.  
>> On X86, the inliner refuses to inline these functions because the 
>> features of the CPU model set on the callee are a superset of those of 
>> the caller.
>> 
>> It is of course possible to compile a short C function with clang and 
>> fish the "target-cpu" attribute out of the IR.  However, I don't want 
>> to introduce another hack.
>> 
>> Can someone recommend a cleaner way to get the datalayout and 
>> "target-cpu" attribute for a generic model of the CPU we're compiling 
>> on?
>> 
>> -Manuel
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