Re: [PATCH v3 2/3] prandom: Convert prandom_u32_state() to __always_inline

David Laight
2026-01-03T12:37:25Z

On Fri, 2 Jan 2026 14:09:26 +0000
Ryan Roberts <ryan.roberts@arm.com> wrote:

> On 02/01/2026 13:39, Jason A. Donenfeld wrote:
> > Hi Ryan,
> >
> > On Fri, Jan 2, 2026 at 2:12 PM Ryan Roberts <ryan.roberts@arm.com> wrote:
> >> context. Given the function is just a handful of operations and doesn't
> >
> > How many? What's this looking like in terms of assembly?
>
> 25 instructions on arm64:
>
> 0000000000000000 <prandom_u32_state>:
> 0: 29401403 ldp w3, w5, [x0]
> 4: aa0003e1 mov x1, x0
> 8: 29410002 ldp w2, w0, [x0, #8]
> c: 531e74a4 lsl w4, w5, #2
> 10: 530e3468 lsl w8, w3, #18
> 14: 4a0400a5 eor w5, w5, w4
> 18: 4a031863 eor w3, w3, w3, lsl #6
> 1c: 53196047 lsl w7, w2, #7
> 20: 53134806 lsl w6, w0, #13
> 24: 4a023442 eor w2, w2, w2, lsl #13
> 28: 4a000c00 eor w0, w0, w0, lsl #3
> 2c: 121b6884 and w4, w4, #0xffffffe0
> 30: 120d3108 and w8, w8, #0xfff80000
> 34: 121550e7 and w7, w7, #0xfffff800
> 38: 120c2cc6 and w6, w6, #0xfff00000
> 3c: 2a456c85 orr w5, w4, w5, lsr #27
> 40: 2a433504 orr w4, w8, w3, lsr #13
> 44: 2a4254e3 orr w3, w7, w2, lsr #21
> 48: 2a4030c2 orr w2, w6, w0, lsr #12
> 4c: 4a020066 eor w6, w3, w2
> 50: 4a050080 eor w0, w4, w5
> 54: 4a0000c0 eor w0, w6, w0
> 58: 29001424 stp w4, w5, [x1]
> 5c: 29010823 stp w3, w2, [x1, #8]
> 60: d65f03c0 ret

That is gcc, clang seems to generate something horrid (from godbolt).
I'm not sure what it has tried to do (and maybe it can't in kernel)
but it clearly doesn't help!
.LCPI0_0:
.word 18
.word 2
.word 7
.word 13
.LCPI0_1:
.word 6
.word 2
.word 13
.word 3
.LCPI0_2:
.word 4294443008
.word 4294967264
.word 4294965248
.word 4293918720
.LCPI0_3:
.word 4294967283
.word 4294967269
.word 4294967275
.word 4294967284
prandom_u32_state:
adrp x9, .LCPI0_1
ldr q0, [x0]
adrp x10, .LCPI0_3
ldr q1, [x9, :lo12:.LCPI0_1]
adrp x9, .LCPI0_0
ldr q3, [x10, :lo12:.LCPI0_3]
ldr q2, [x9, :lo12:.LCPI0_0]
adrp x9, .LCPI0_2
mov x8, x0
ushl v1.4s, v0.4s, v1.4s
ushl v2.4s, v0.4s, v2.4s
eor v0.16b, v1.16b, v0.16b
ldr q1, [x9, :lo12:.LCPI0_2]
and v1.16b, v2.16b, v1.16b
ushl v0.4s, v0.4s, v3.4s
orr v0.16b, v0.16b, v1.16b
ext v1.16b, v0.16b, v0.16b, #8
str q0, [x8]
eor v1.8b, v0.8b, v1.8b
fmov x9, d1
lsr x10, x9, #32
eor w0, w9, w10
ret

The x86 versions are a little longer (arm's barrel shifter helps a lot).

>
> > It'd also be
> > nice to have some brief analysis of other call sites to have
> > confirmation this isn't blowing up other users.
>
> I compiled defconfig before and after this patch on arm64 and compared the text
> sizes:
>
> $ ./scripts/bloat-o-meter -t vmlinux.before vmlinux.after
> add/remove: 3/4 grow/shrink: 4/1 up/down: 836/-128 (708)
> Function old new delta
> prandom_seed_full_state 364 932 +568
> pick_next_task_fair 1940 2036 +96
> bpf_user_rnd_u32 104 196 +92
> prandom_bytes_state 204 260 +56
> e843419@0f2b_00012d69_e34 - 8 +8
> e843419@0db7_00010ec3_23ec - 8 +8
> e843419@02cb_00003767_25c - 8 +8
> bpf_prog_select_runtime 448 444 -4
> e843419@0aa3_0000cfd1_1580 8 - -8
> e843419@0aa2_0000cfba_147c 8 - -8
> e843419@075f_00008d8c_184 8 - -8
> prandom_u32_state 100 - -100
> Total: Before=19078072, After=19078780, chg +0.00%
>
> So 708 bytes more after inlining.

Doesn't look like there are many calls.

> The main cost is prandom_seed_full_state(),
> which calls prandom_u32_state() 10 times (via prandom_warmup()). I expect we
> could turn that into a loop to reduce ~450 bytes overall.

That would always have helped the code size.
And I suspect the other costs of that code make unrolling the loop pointless.

>
> I'm not really sure if 708 is good or bad...
>
> >
> >> +static __always_inline u32 prandom_u32_state(struct rnd_state *state)
> >
> > Why not just normal `inline`? Is gcc disagreeing with the inlinability
> > of this function?
>
> Given this needs to be called from a noinstr function, I didn't want to give the
> compiler the opportunity to decide not to inline it, since in that case, some
> instrumentation might end up being applied to the function body which would blow
> up when called in the noinstr context.
>
> I think the other 2 options are to keep prandom_u32_state() in the c file but
> mark it noinstr or rearrange all the users so that thay don't call it until
> instrumentation is allowable. The latter is something I was trying to avoid.
>
> There is some previous discussion of this at [1].
>
> [1] https://lore.kernel.org/all/aS65LFUfdgRPKv1l@J2N7QTR9R3/
>
> Perhaps keeping prandom_u32_state() in the c file and making it noinstr is the
> best compromise?

Or define prandom_u32_state_inline() as always_inline and have the
real function:
u32 prandom_u32_state(struct rnd_state *state)
{
return prandom_u32_state_inline(state);
}

So that the callers can pick the inline version if it really matters.

David

>
> Thanks,
> Ryan
>
> >
> > Jason
>
>