RE: Address Arithmetic Improvement

RE: Address Arithmetic Improvement

[Rakesh Kumar - Software, Noida]

Hi,

> I was thinking about this problem a few weeks ago, and it may be possible
> to use the peep2 pass to opportunistically combine multiple adds into a
> single add instruction, such as

peep2 works for adjacent instructions only. Defining one peep2 
will not suffice for all cases. For instance, in your example:

	fmov.s	@r0+,fr0
	fmov.s	@r0,fr1
	add	#-4,r0
...						<-- Here
	add	#64,r0
	fmov.s	@r0+,fr2
	fmov.s	@r0,fr3

At the location marked Here, there can be multiple instructions. Now, if we
are considering a peep window of say 4 instructions, we need to define peep2
for 3 cases eventually, naming

1.	add #-4, r0
	add #64, r0

2.	add #-4, r0
	one insn not ending the basic block and not using r0 (use
match_insn)
	add #64, r0

3.	add #-4, r0
	two insns with same constraints
	add #64, r0

As we increase the peephole window, we need to increase the number of
peepholes as well.

It doesn't seem that it solves the problem since there might be instructions
which 
may not come in that window but may exist in the same basic block.
As the insns are split during the postrelad pass, there is possibility of
such cases.

I suppose there are two solutions to this problem:

1. We could develop a new pass that improves the address arithmetic. 
   The disadvantage of this is the increased compile time.

2. One more cleaner solution is to improve the implementation of -fmovd on
SH4.
   This flag is set for making double-sized floating point moves but at
present it is not 
   supported properly. If set, this should not eventually cause those DF
splits and no more   
   address arithmetic instructions are generated.

What do you think?

Thanks and Best Regards,
Rakesh Kumar

Re: Address Arithmetic Improvement

[tm_gccmail]

On Wed, 9 Apr 2003, Rakesh Kumar - Software, Noida wrote:

> Processor: SH4
> Mode: Little Endian
>  
> On compiling the attached program with -O2, I observed that reload pass
> splits up the DF mode address arithmetic insn into SF mode insns using FPSCR
> register.
>  
> I believe SH4 has the restriction that in little endian, we can't use
> double-precision floating-point moves between memory and registers.
>  
> But generating two insns, where one would suffice, encourage redundancy.
> Also to adjust the pointers, we are generating additional insns as defined
> in define_split in sh.md.
>  
> I propose that one function could be developed which scans the basic block
> and combines address arithmetic insns. But I'm not clear about where to put
> it so that it should not affect the present compilation process.
>  
> Can anybody there suggest me some alternative?
>  
> Thanks in advance
> Rakesh Kumar
> 

I think regmove is supposed to do this, but it runs too early to catch the
post-reload DFmode splits.

I was thinking about this problem a few weeks ago, and it may be possible
to use the peep2 pass to opportunistically combine multiple adds into a
single add instruction, such as:

	fmov.s	@r0+,fr0
	fmov.s	@r0,fr1
	add	#-4,r0
...
	add	#64,r0
	fmov.s	@r0+,fr2
	fmov.s	@r0,fr3

into

	fmov.s	@r0+,fr0
	fmov.s	@r0,fr1
...
	add	#60,r0
	fmov.s	@r0+,fr2
	fmov.s	@r0,fr3

Toshi

Address Arithmetic Improvement

[Rakesh Kumar - Software, Noida]

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Processor: SH4
Mode: Little Endian
 
On compiling the attached program with -O2, I observed that reload pass
splits up the DF mode address arithmetic insn into SF mode insns using FPSCR
register.
 
I believe SH4 has the restriction that in little endian, we can't use
double-precision floating-point moves between memory and registers.
 
But generating two insns, where one would suffice, encourage redundancy.
Also to adjust the pointers, we are generating additional insns as defined
in define_split in sh.md.
 
I propose that one function could be developed which scans the basic block
and combines address arithmetic insns. But I'm not clear about where to put
it so that it should not affect the present compilation process.
 
Can anybody there suggest me some alternative?
 
Thanks in advance
Rakesh Kumar

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[-- Attachment #2: address.c --]
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#include <stdarg.h>

void func(char *, ...);

void func2(int *a, double *b)
{
   func(".", a[0], b[0], a[1], b[1], a[2], b[2], a[3], b[3], a[4], b[4]);
}