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

[Eugene Loh]

On 3/18/22 6:54 PM, Kris Van Hees wrote:

> On Fri, Mar 18, 2022 at 04:30:13PM -0400, Eugene Loh via DTrace-devel wrote:
>> On 3/18/22 3:04 PM, Kris Van Hees via DTrace-devel wrote:
>>> diff --git a/libdtrace/dt_cg.c b/libdtrace/dt_cg.c
>>> +	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.
> We should be avoiding crossing lines with alloc/free of registers,
Why?  This isn't a stack/LIFO situation.

Further, we do this all the time.  E.g., we alloc regs for the func 
call, then we alloc dnp->dn_reg, then we free the func-call regs, and 
later we will free dnp->dn_reg.  We do this sort of thing routinely.

Also, what about the scenario I raised?  E.g., let's say dnp->dn_reg is 
assigned %r5.  After all, the situation is that we are facing register 
pressure.  So:

*)  alloc %r1-%r5
*)  alloc %r0
*)  call func (%r0 is given a value while %r1-%r5 are clobbered)
*)  alloc dnp->dn_reg, getting %r5, whose clobbered value is spilled to 
stack
*)  %r5 = %r0
*)  free %r0
*)  free %r1-%r5, filling %r5 (dnp->dn_reg) with the clobbered value
> so if we
> alloc the args first and then %r0, we should free %r0 first, and then the args.