optimization issues in gcc 4,8

optimization issues in gcc 4,8

[Johan Danielsson]

Hello,

I'm looking at moving GCC from 4.5 to 4.8 for a custom CPU. For the
most part GCC 4.8 produces better code (smaller, which is mostly what
matters in my case). There are however some parts where it does
worse. Trying to isolate some of these cases I've come across the
following.

I have the this function (which is really part of a larger context),
multiplying two signed 32-bit integers returning a 64-bit signed
integer:

long long mul_32_64(int a, int b)
{
   unsigned int lo;
   unsigned int hi;

   __asm__("muls.w   %0, %2, %3\n"
           "mulshi.w %1, %2, %3"
           : "=&r"(lo), "=r"(hi)
           : "r"(a), "r"(b));

   return (long long)(((unsigned long long)hi << 32) | lo);
}

In this CPU r0..r3 are used for arguments, and are also temporary
registers.

GCC 4.5.3 (-Os) produces the following assembler:

mul_32_64:
        psh.h    1,4            ; save r4
        mov.h    r3,r0
        muls.w   r4, r1, r2     ; mul a and b, put result in r4
        mulshi.w r2, r1, r2     ; ... and the upper half in r2
        sw.h     0(r3),r4       ; store low part to return value
        sw.h     4(r0),r2       ; ... and high
        popr.h   1,4            ; restore r4 and return

This is close to optimal, except that if it had used r3 instead of r4
it would not have had to save any registers at all. GCC 4.7.3 gives
the same result.

Now this is what GCC 4.8.2 thinks about this:

mul_32_64:
        psh.h    6,24
        mov.h    r3,r0
        muls.w   r4, r1, r2
        mulshi.w r1, r1, r2
        mov.h    r2,r4
        ldi.h    r4,0
        mov.h    r5,r4
        mov.h    r4,r1
        mov.h    r6,r5
        mov.h    r7,r1
        mov.h    r9,r5
        mov.h    r8,r2
        sw.h     0(r3),r2
        sw.h     4(r3),r1
        popr.h   6,24

Clearly it has given up on optimizing this. Looking at the RTL output,
the step that removes all register shuffling is fwprop, but for 4.8
this does nothing.

So my question is really what is causing this. Is it a bug in GCC 4.8,
is there some misconfiguration in the backend that is only hitting
some functions, or something else. How do I debug this?

/Johan