[PATCH 2/4] RISC-V: reduce mcount save space on RV32

[Andrew Jones]

On Fri, Oct 28, 2022 at 04:49:25PM +0200, Andrew Jones wrote:
> On Thu, Oct 27, 2022 at 06:24:33PM +0100, Jamie Iles wrote:
> > For RV32 we can reduce the size of the ABI save+restore state by using
> > SZREG so that register stores are packed rather than on an 8 byte
> > boundary.
> > 
> > Signed-off-by: Jamie Iles <jamie at jamieiles.com>
> > ---
> >  arch/riscv/kernel/mcount.S | 32 ++++++++++++++++----------------
> >  1 file changed, 16 insertions(+), 16 deletions(-)
> > 
> > diff --git a/arch/riscv/kernel/mcount.S b/arch/riscv/kernel/mcount.S
> > index 9cf0904afd6d..4166909ce7b3 100644
> > --- a/arch/riscv/kernel/mcount.S
> > +++ b/arch/riscv/kernel/mcount.S
> > @@ -15,8 +15,8 @@
> >  
> >  	.macro SAVE_ABI_STATE
> >  	addi	sp, sp, -16
> > -	REG_S	s0, 0(sp)
> > -	REG_S	ra, 8(sp)
> > +	REG_S	s0, 0*SZREG(sp)
> > +	REG_S	ra, 1*SZREG(sp)
> >  	addi	s0, sp, 16
> >  	.endm
> 
> Shouldn't we only be adjusting sp by 2*SZREG?. Indeed RESTORE_ABI_STATE
> below is getting modified that way.

It just occurred to me that leaving this adjustment as 16 is correct,
since we need to maintain 16-byte alignment, so the issue is below
in RESTORE_ABI_STATE where it isn't adding back 16.

> 
> >  
> > @@ -25,24 +25,24 @@
> >  	 * register if a0 was not saved.
> >  	 */
> >  	.macro SAVE_RET_ABI_STATE
> > -	addi	sp, sp, -32
> > -	REG_S	s0, 16(sp)
> > -	REG_S	ra, 24(sp)
> > -	REG_S	a0, 8(sp)
> > +	addi	sp, sp, -4*SZREG
> > +	REG_S	s0, 2*SZREG(sp)
> > +	REG_S	ra, 3*SZREG(sp)
> > +	REG_S	a0, 1*SZREG(sp)
> >  	addi	s0, sp, 32
> 
> This should be 'addi s0, sp, -4*SZREG'

And here I made copy+paste mistake, it should of course be 4*SZREG.

Thanks,
drew

> 
> >  	.endm
> >  
> >  	.macro RESTORE_ABI_STATE
> > -	REG_L	ra, 8(sp)
> > -	REG_L	s0, 0(sp)
> > -	addi	sp, sp, 16
> > +	REG_L	ra, 1*SZREG(sp)
> > +	REG_L	s0, 0*SZREG(sp)
> > +	addi	sp, sp, 2*SZREG
> >  	.endm
> >  
> >  	.macro RESTORE_RET_ABI_STATE
> > -	REG_L	ra, 24(sp)
> > -	REG_L	s0, 16(sp)
> > -	REG_L	a0, 8(sp)
> > -	addi	sp, sp, 32
> > +	REG_L	ra, 3*SZREG(sp)
> > +	REG_L	s0, 2*SZREG(sp)
> > +	REG_L	a0, 1*SZREG(sp)
> > +	addi	sp, sp, 4*SZREG
> >  	.endm
> >  
> >  ENTRY(ftrace_stub)
> > @@ -101,10 +101,10 @@ ENTRY(MCOUNT_NAME)
> >   * prepare_to_return(&ra_to_caller_of_caller, ra_to_caller)
> >   */
> >  do_ftrace_graph_caller:
> > -	addi	a0, s0, -8
> > +	addi	a0, s0, -SZREG
> >  	mv	a1, ra
> >  #ifdef HAVE_FUNCTION_GRAPH_FP_TEST
> > -	REG_L	a2, -16(s0)
> > +	REG_L	a2, -2*SZREG(s0)
> >  #endif
> >  	SAVE_ABI_STATE
> >  	call	prepare_ftrace_return
> > @@ -117,7 +117,7 @@ do_ftrace_graph_caller:
> >   * (*ftrace_trace_function)(ra_to_caller, ra_to_caller_of_caller)
> >   */
> >  do_trace:
> > -	REG_L	a1, -8(s0)
> > +	REG_L	a1, -SZREG(s0)
> >  	mv	a0, ra
> >  
> >  	SAVE_ABI_STATE
> > -- 
> > 2.34.1
> >
> 
> Thanks,
> drew