shell: move position of last_nl in struct shell_backend_ctx_flags

[maass-hamburg]

last_nl is 8 bits long, put it at the beginning of the packed struct, so it can be read and written with one intruction, as it is now aligned.

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud

[jakub-uC]

Thanks for the suggestion! Just a quick note: this is a bitfield within a uint32_t, not a packed struct, so the compiler will likely handle it as a single word access regardless of the order. Do you happen to have any benchmarks or assembly output showing that this specific alignment improves efficiency? I suspect the impact might be negligible here.

[maass-hamburg]

for RISCV:

before:

	return sh->ctx->ctx.flags.last_nl;
40009bd8:	29072683          	lw	a3,656(a4)
40009bdc:	0036d693          	srli	a3,a3,0x3
40009be0:	0ff6f693          	zext.b	a3,a3
	if ((z_flag_last_nl_get(sh) == 0U) ||
40009be4:	00068a63          	beqz	a3,40009bf8 <state_collect+0x210>
40009be8:	29072683          	lw	a3,656(a4)
40009bec:	0036d693          	srli	a3,a3,0x3
40009bf0:	0ff6f693          	zext.b	a3,a3
40009bf4:	f6d794e3          	bne	a5,a3,40009b5c <state_collect+0x174>
	sh->ctx->ctx.flags.last_nl = val;
40009bf8:	00379693          	slli	a3,a5,0x3
40009bfc:	29075783          	lhu	a5,656(a4)
40009c00:	8077f793          	andi	a5,a5,-2041
40009c04:	00d7e7b3          	or	a5,a5,a3
40009c08:	28f71823          	sh	a5,656(a4)

after:

	return sh->ctx->ctx.flags.last_nl;
40009afc:	28874683          	lbu	a3,648(a4)
	if ((z_flag_last_nl_get(sh) == 0U) ||
40009b00:	00068663          	beqz	a3,40009b0c <shell_process+0x224>
40009b04:	28874683          	lbu	a3,648(a4)
40009b08:	f6d79ce3          	bne	a5,a3,40009a80 <shell_process+0x198>
	sh->ctx->ctx.flags.last_nl = val;
40009b0c:	28f70423          	sb	a5,648(a4)

build cmd:
west build -p -b litex_vexriscv samples/subsys/shell/shell_module/ -- -DCONFIG_OUTPUT_DISASSEMBLY=y

As far as I know a compiler would never change the order of the members of a struct itself.