[tree-ssa] RFC: Dropping INDIRECT_REF variables

[Timothy J. Wood]

On Wednesday, May 14, 2003, at 08:09  AM, Diego Novillo wrote:
> - We disable the ability to treat non-aliased pointer
>   dereferences as if they were variables.  We would lose the
>   ability to do some optimizations like:

   Please forgive me if I'm way off base here, but one example on PPC 
that your code snippet reminded me of was int->float conversion.  On 
PPC this happens by building storing a pair of 32-bit constructed ints 
on the stack, loading them as a double and then doing some more 
contortions with this.

   The current problem is that if you have float conversion in a loop:

void convert(int *input, float *output, unsigned int count)
{
     while (count--)
         *output++ = *input++;
}

   both words of the double on the stack are written on each loop even 
though the one 32-bit portion is always the same.  This store should be 
hoisted outside the loop (in the degenerate case above this can by a 
pretty big performance win).  I don't have a 3.3 compiler right now, 
but on Mac OS X 10.2 (3.1-based):

cc -mdynamic-no-pic -S -O2 foo.c

.data
.literal8
         .align 3
LC0:
         .long   1127219200
         .long   -2147483648
.text
         .align 2
         .globl _convert
_convert:
         cmpwi cr0,r5,0
         addi r5,r5,-1
         beqlr- cr0
         addi r5,r5,1
         lis r11,ha16(LC0)
         mtctr r5
         lfd f13,lo16(LC0)(r11)
         lis r9,0x4330
L8:
         lwz r0,0(r3)
         addi r3,r3,4
         stw r9,-16(r1)  <--- should be outside the loop
         xoris r0,r0,0x8000
         stw r0,-12(r1)
         lfd f0,-16(r1)
         fsub f0,f0,f13
         frsp f0,f0
         stfs f0,0(r4)
         addi r4,r4,4
         bdnz L8
         blr

   I'm not sure this applies to what you were talking about, but I 
thought I'd bring it up since it looks similar and we could benefit 
from having this optimization.

-tim