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

Manuel Jacob via llvm-dev
Fri, 15 Jan 2016 01:31:20 -0000

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