Re: constant hoisting out of loops

[Jim Wilson <wilson at codesourcery dot com>]

On 03/10/2010 10:48 PM, fanqifei wrote:
> For below piece of code, the instruction "clr.w a15" obviously doesn't
> belong to the inner loop.
>     6:   bd f4            clr.w a15; #clear to zero
>     8:   80 af 00        std.w a10 0x0 a15;

There is info lacking here.  Did you compile with optimization?  What 
does the RTL look like before and after the loop opt passes?

I'd guess that your movsi pattern is defined wrong.  You probably have 
predicates that allow either registers or constants in the set source, 
which is normal, and constraints that only allow registers when the dest 
is a mem.  But constraints are only used by the reload pass, so a store 
zero to mem rtl insn will be generated early, and then fixed late during 
the reload pass.  So the loop opt did not move the clear insn out of the 
loop because there was no clear insn at this time.

The way to fix this is to add a condition to the movsi pattern that 
excludes this case.  For instance, something like this:
   "(register_operand (operands[0], SImode)
     || register_operand (operands[1], SImode))"
This will prevent a store zero to mem RTL insn from being accepted.  In 
order to make this work, you need to make movsi an expander that accepts 
anything, and then forces the source to a register if you have a store 
constant to memory.  See for instance the sparc_expand_move function or 
the mips_legitimize_move function.

Use -da (old) or -fdump-rtl-all (new) to see the RTL dumps to see what 
is going on.

Jim