Re: [arm] Fix thumb register allocation failure

[Richard Earnshaw <rearnsha at arm dot com>]

On Wed, 2004-04-07 at 21:09, Paul Brook wrote:
> The patch below fixes a register allocation failure when compiling for thumb.
> When the code below is compiled with gcc -mthumb -O2 -fno-omit-frame pointer 
> combine keep the argument registers (r0-r3) live up until the call to z. r7 
> is use for the frame pointer, which leaves only 3 register available. This is 
> insufficient to perform the doubleword add.
> 
> The fix is make CLASS_LIKELY_SPILLED_P return true for LO_REGS.
> 
> Tested with cross-compiler to arm-none-elf.
> Ok?
> 
> Paul
> 
> void z(int, int *); 
> int foo(int a, int b, int c, int d, long long *q) 
> { 
>   int p; 
>   *q=*q+1; 
>   z (a+b+c+d, &p); 
> }
> 
> 2004-04-07  Paul Brook  <paul@codesourcery.com>
> 
> 	* arm.h (CLASS_LIKELY_SPILLED_P): Define.

Note that CC_REG class registers are always used as fixed registers (not
available to the register allocator), so I'd be surprised if it was
necessary to check that.

I suppose this patch is necessary even though it will probably pessimize
code.  There's been a large jump in code size (particularly at -O2 --
0.5%) in the CSiBE benchmark at about the time this patch went in.  Is
this patch responsible, or do I need to hunt for some other culprit?

R.