[llvm-dev] IPRA, interprocedural register allocation, question

vivek pandya via llvm-dev llvm-dev at lists.llvm.org
Wed Jul 6 14:09:14 PDT 2016

Hello Peter,

Thanks to pointing out this interesting case.

> Vivek,
>           I have an application where many of the leaf functions are
> Hand-coded assembly language,  because they use special IO instructions
> That only the assembler knows about.  These functions typically don't
> Use any registers besides the incoming argument registers, IE they don't
> Need to use any additional callee-save nor caller-save registers.
> If inline asm template has specified clobbered list properly than IPRA is
able to use that information and it propagates correct register mask (and
that also means that skipping clobbers list while IPRA enabled may broke
For example in following code:
int gcd( int a, int b ) {
    int result ;
    /* Compute Greatest Common Divisor using Euclid's Algorithm */
    __asm__ __volatile__ ( "movl %1, %%r15d;"
                          "movl %2, %%ecx;"
                          "CONTD: cmpl $0, %%ecx;"
                          "je DONE;"
                          "xorl %%r13d, %%r13d;"
                          "idivl %%ecx;"
                          "movl %%ecx, %%r15d;"
                          "movl %%r13d, %%ecx;"
                          "jmp CONTD;"
                          "DONE: movl %%r15d, %0;" : "=g" (result) : "g"
(a), "g" (b) : "ecx" ,"r13", "r15"

    return result ;
IPRA calculates and propagates correct regmask in which it marks CH, CL,
ECX .. clobbered and R13, R15 is not marked clobbered as it is callee saved
and LLVM code generators also insert spill/restores code for them.

Is there any way in your IPRA interprocedural register allocation project
> that
> The user can supply this information for external functions ?

By external word do you here mean function defined in other module than
being used?  In that case as IPRA can operate on only one module at time
register usage propagation is not possible. But there is a work around for
this problem. You can use IPRA with link time optimization enabled because
the way LLVM LTO works it creates a big IR modules out of source files and
them optimize and codegen it so in that case IPRA can have actual register
usage info (if function will be compiled in current module).

In case you want to experiment with IPRA please apply
http://reviews.llvm.org/D21395 this patch before you begin.



Perhaps using some form of __attribute__ ?
> Maybe __attribute__ ((registermask = ....))  ?
> --Peter Lawrence.
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