[Bug target/55372] New: MIPS: Loading integer constants to floating-pointer registers generates suboptimal code

[stevenbaker94 at rocketmail dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=55372



             Bug #: 55372

           Summary: MIPS: Loading integer constants to floating-pointer

                    registers generates suboptimal code

    Classification: Unclassified

           Product: gcc

           Version: 4.7.2

            Status: UNCONFIRMED

          Severity: enhancement

          Priority: P3

         Component: target

        AssignedTo: unassig...@gcc.gnu.org

        ReportedBy: stevenbake...@rocketmail.com





When loading an integer constant (e.g. 100) into a floating-point variable, the

compiler unconditionally creates a .rodata entry. However, integer constants

that are to be used in floating-point registers always have the lower 16 bits

set to 0, so it is better to use the combination lui+mtc1 rather than lui+lwc1

and a .data entry. (In other words, we save both a word of memory in .data and

one memory fetch for this word.)



As a workaround, I provide a function f() that "converts" a const float into

float using the right instruction sequence (beware that it silently discards

any fractional bits, however):



extern void foo(float x);



static inline float f(const float f)

{

        union {

                float f;

                unsigned int i;

        } x;



        x.f = f;



        unsigned int r;

        float f_out;

        asm ("lui %0, %1"

                : "=d" (r)

                : "I" (x.i >> 16));

        asm ("mtc1 %1, %0"

                : "=f" (f_out)

                : "d" (r));

        return f_out;

}



void bar()

{

#if 1 /* Workaround */

        foo(f(100.0f));

#else /* Native GCC */

        foo(100.0f);

#endif

}



The difference:



ORIGINAL:



Contents of section .rodata.cst4:

 0000 42c80000                             B...            



00000000 <bar>:

   0:   3c020000        lui     v0,0x0

   4:   08000000        j       0 <bar>

   8:   c44c0000        lwc1    $f12,0(v0)



WORKAROUND/PROPOSED IMPROVEMENT:



00000000 <bar>:

   0:   3c0242c8        lui     v0,0x42c8

   4:   08000000        j       0 <bar>

   8:   44826000        mtc1    v0,$f12