Build core, alloc, std with 1 CGU

[erikdesjardins]

Locally, this makes helloworld ~24% smaller:

Stats for benchmark `helloworld`
--------------------
Target `helloworld` (artifact `helloworld`)
┌─────────────────────┬───────────────┬──────────────┬─────────┬──────────┐
│ Section             │ Size (before) │ Size (after) │    Diff │ Diff (%) │
├─────────────────────┼───────────────┼──────────────┼─────────┼──────────┤
│ .strtab             │     98.78 KiB │    52.35 KiB │  -47543 │   -47.0% │
│ .text               │    276.83 KiB │   250.88 KiB │  -26576 │    -9.4% │
│ .eh_frame           │     42.02 KiB │    22.45 KiB │  -20048 │   -46.6% │
│ .symtab             │     37.69 KiB │    18.75 KiB │  -19392 │   -50.2% │
│ .gcc_except_table   │     11.00 KiB │     4.33 KiB │   -6824 │   -60.6% │
│ .rodata             │     28.42 KiB │    24.43 KiB │   -4083 │   -14.0% │
│ .eh_frame_hdr       │      8.16 KiB │     4.19 KiB │   -4064 │   -48.6% │
│ .rela.dyn           │     20.37 KiB │    17.06 KiB │   -3384 │   -16.2% │
│ .data.rel.ro        │     10.56 KiB │     9.05 KiB │   -1544 │   -14.3% │
│ .got                │      1.98 KiB │     1.66 KiB │    -328 │   -16.2% │
│ .dynstr             │      1.04 KiB │     1.02 KiB │     -25 │    -2.3% │
│ .dynsym             │      1.59 KiB │     1.57 KiB │     -24 │    -1.5% │
│ .gnu.version_r      │         288 B │        272 B │     -16 │    -5.6% │
│ .bss                │         200 B │        216 B │     +16 │    +8.0% │
│ .gnu.version        │         136 B │        134 B │      -2 │    -1.5% │
│ .tbss               │          81 B │         80 B │      -1 │    -1.2% │
│ <16 unchanged rows> │      1.28 KiB │     1.28 KiB │       0 │     0.0% │
│─────────────────────│───────────────│──────────────│─────────│──────────│
│ Total               │    540.41 KiB │   409.71 KiB │ -133838 │   -24.2% │
└─────────────────────┴───────────────┴──────────────┴─────────┴──────────┘

It also makes ./x build --stage 1 ~10% slower after a change to the stdlib (2:45 -> 3:00).

I'm not sure if that tradeoff is worthwhile.

Let's do a perf run first.

r? @ghost

[erikdesjardins]

(I hope this isn't just a duplicate of #115554, and the only reason it seems like an improvement is because that flag doesn't get set unless you're on CI. Although that change didn't have any binary size improvements on compile benchmarks so it seems like it only applies to rustc itself, not std.)

[lqd]

Did you run your benchmark on a local build with the rust.codegen-units-std=1 config? This should be the setting we currently use to build the std on the main targets.

rust.codegen-units=1 is indeed set on CI so that local builds are not slowed down too much. IIRC #115554 didn’t have binary size improvements because we were already building the std with 1 CGU.

[erikdesjardins]

Ah, that's what I was missing. Yeah, this has no effect.