[DTrace-devel] [PATCH 01/10] Reduce register pressure in substr()

[Eugene Loh]

Reviewed-by: Kris Van Hees <kris.van.hees at oracle.com>
with some questions and comments...

On 3/18/22 3:04 PM, Kris Van Hees via DTrace-devel wrote:
> Have substr() return the result string pointer.  This means that we can
> delay allocating the register to hold the result until after the call to
> dt_substr() has been made.
Good idea but this raises the question whether we should be doing this 
sort of thing more universally.  Or do we do this on a case-by-case 
basis... "reactively"?  Is it worth having a test case for this instance?
> Signed-off-by: Kris Van Hees <kris.van.hees at oracle.com>
> ---
>   bpf/substr.S      |  3 ++-
>   libdtrace/dt_cg.c | 29 ++++++++++++++++-------------
>   2 files changed, 18 insertions(+), 14 deletions(-)
>
> diff --git a/bpf/substr.S b/bpf/substr.S
> index 8d501faa..9bd2400d 100644
> --- a/bpf/substr.S
> +++ b/bpf/substr.S
> @@ -6,7 +6,7 @@
>   #define BPF_FUNC_probe_read_str	45
>   
>   /*
> - * void dt_substr(char *dst, const char *src, int32_t idx, int32_t cnt,
> + * char *dt_substr(char *dst, const char *src, int32_t idx, int32_t cnt,
>    *		  uint64_t argc)
>    *
>    * %r1 = dst, %r2 = src, %r3 = idx, %r4 = cnt, %r5 = argc
> @@ -124,6 +124,7 @@ dt_substr :
>   					 *		       &src[idx]);
>   					 */
>   
> +	mov	%r0, %r9
>   	exit
>   
>   .Lempty:

I confess I do not understand why there is no similar %r0=%r9 on the 
.Lempty code path.  If we jump to .Lempty, %r0 is "not properly" set and 
then we return.  The clause function then picks this %r0 up and goes on 
its way.  That seems to me to be wrong, but I cannot demonstrate an 
error as easily as I would have thought.  Is the %r0=%r9 not needed on 
.Lempty?

> diff --git a/libdtrace/dt_cg.c b/libdtrace/dt_cg.c
> index 6f95260c..2cf48994 100644
> --- a/libdtrace/dt_cg.c
> +++ b/libdtrace/dt_cg.c
> @@ -3997,26 +3997,20 @@ dt_cg_subr_substr(dt_node_t *dnp, dt_irlist_t *dlp, dt_regset_t *drp)
>   		dt_cg_node(cnt, dlp, drp);
>   
>   	/*
> -	 * Allocate the result register and associate it with a temporary
> -	 * string slot.
> +	 * Allocate a temporary string slot for the result.
>   	 */
> -	dnp->dn_reg = dt_regset_alloc(drp);
> -	if (dnp->dn_reg == -1)
> -		longjmp(yypcb->pcb_jmpbuf, EDT_NOREG);
>   	dt_cg_tstring_alloc(yypcb, dnp);
>   
> -        emit(dlp,  BPF_LOAD(BPF_DW, dnp->dn_reg, BPF_REG_FP, DT_STK_DCTX));
> -        emit(dlp,  BPF_LOAD(BPF_DW, dnp->dn_reg, dnp->dn_reg, DCTX_MEM));
> -        emit(dlp,  BPF_ALU64_IMM(BPF_ADD, dnp->dn_reg, dnp->dn_tstring->dn_value));
> -
>   	if (dt_regset_xalloc_args(drp) == -1)
>   		longjmp(yypcb->pcb_jmpbuf, EDT_NOREG);
>   
> -	emit(dlp, BPF_MOV_REG(BPF_REG_1, dnp->dn_reg));
> -	emit(dlp, BPF_MOV_REG(BPF_REG_2, str->dn_reg));
> +        emit(dlp,  BPF_LOAD(BPF_DW, BPF_REG_1, BPF_REG_FP, DT_STK_DCTX));
> +        emit(dlp,  BPF_LOAD(BPF_DW, BPF_REG_1, BPF_REG_1, DCTX_MEM));
> +        emit(dlp,  BPF_ALU64_IMM(BPF_ADD, BPF_REG_1, dnp->dn_tstring->dn_value));
Might as well change those spaces to tabs since this code is being 
touched anyhow.
> +	emit(dlp,  BPF_MOV_REG(BPF_REG_2, str->dn_reg));
>   	dt_regset_free(drp, str->dn_reg);
>   	dt_cg_tstring_free(yypcb, str);
> -	emit(dlp, BPF_MOV_REG(BPF_REG_3, idx->dn_reg));
> +	emit(dlp,  BPF_MOV_REG(BPF_REG_3, idx->dn_reg));
>   	dt_regset_free(drp, idx->dn_reg);
>   	if (cnt != NULL) {
>   		emit(dlp, BPF_MOV_REG(BPF_REG_4, cnt->dn_reg));
> @@ -4030,8 +4024,17 @@ dt_cg_subr_substr(dt_node_t *dnp, dt_irlist_t *dlp, dt_regset_t *drp)
>   	idp = dt_dlib_get_func(yypcb->pcb_hdl, "dt_substr");
>   	assert(idp != NULL);
>   	emite(dlp,  BPF_CALL_FUNC(idp->di_id), idp);
> -	dt_regset_free_args(drp);
> +
> +	/*
> +	 * Allocate the result register, and assign the result to it..
> +	 */
> +	dnp->dn_reg = dt_regset_alloc(drp);
> +	if (dnp->dn_reg == -1)
> +		longjmp(yypcb->pcb_jmpbuf, EDT_NOREG);
> +
> +	emit(dlp, BPF_MOV_REG(dnp->dn_reg, BPF_REG_0));
>   	dt_regset_free(drp, BPF_REG_0);
> +	dt_regset_free_args(drp);
I think the dt_regset_free_args(drp) call should remain tight up against 
the BPF_CALL_FUNC().  After all, as soon as that function call is made, 
the regs are toast.  Or, say dnp->dn_reg is assigned one of %r1-%r5 and 
therefore spills that register.  (Does that cause the BPF verifier to 
complain?)  Then we free_args() and fill the register, overwriting what 
was just there.

>   	TRACE_REGSET("    subr-substr:End  ");
>   }