[llvm-dev] Is there any relationship between IR instruction and execution time

[Yin Liu via llvm-dev]

Hi Matt,

Thanks you so much for the reply!

I've tried the llvm-mca, it is helpful.
I was wondering whether the llvm-mca support the assembly code for the ARM?

I cross-compile the test file for ARM like that: clang test.c -O2 -target
arm-linux-gnueabihf -static -S -o test.s

If I want to check the performance using llvm-mca, is there any option of
"-mcpu" for ARM ?


Thanks,
Yin


2018-05-07 21:52 GMT-04:00 <Matthew.Davis at sony.com>:

> Hi Yin,
>
> From: llvm-dev <llvm-dev-bounces at lists.llvm.org> On Behalf Of Yin Liu via
> llvm-dev
> > Hello,
> >
> > As is known to all, there is a relationship between program's
> instructions and its execution time. In other words, we can estimate the
> execution time based on the number of program > instructions.
> >
> > I'm curious about what the relationship between IR instruction and
> execution time. I know the number of program instructions and the execution
> time is highly related to the
> > platform and architecture, while the IR instruction is independent and
> intermediate. But, intuitively, there may be some relationship between IR
> instruction and execution time.
> >
> > Would it be possible to give me some advice about it?
>
> What instructions finally get emitted by the compiler is highly dependent
> on the specified target.  As you pointed out, IR is relatively abstract,
> and can at best only generate a "rough" estimate to timing.  Maybe that
> loss of fidelity is acceptable in your case.  Be aware that there are also
> target specific optimizations that operate after the IR is lowered to a
> target friendly representation.  Any early approximation of IR performance
> will be less accurate after target specific optimization passes are ran.
> For more accurate results, you will need to wait until the IR is lowered to
> the target architecture and emitted as assembly or object code.  But it
> seems that might be too late for what you are looking for.  In any case, if
> you do want to analyze the assembly code, then look no further than llvm's
> Machine Code Analyzer(MCA).  This tool takes an assembly code as input and
> generates throughput and latency information.  For more details see:
> https://llvm.org/docs/CommandGuide/llvm-mca.html
>
> -Matt
>
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