Always generate GEP i8 / ptradd for struct offsets

[erikdesjardins]

This implements #98615, and goes a bit further to remove struct_gep entirely.

Upstream LLVM is in the beginning stages of migrating to ptradd. LLVM 19 will canonicalize all constant-offset GEPs to i8, which has roughly the same effect as this change.

Fixes #121719.

Split out from #121577.

r? @nikic

[rustbot]

Some changes occurred in compiler/rustc_codegen_gcc

cc @antoyo, @GuillaumeGomez

[erikdesjardins]

Normally I'd feel bad about this kind of manual layout computation, but this file is already packed full of hardcoded offsets and assumptions.

At some point we should re-evaluate whether we can use the LLVM va_arg directly.

To that point, I'm confused about the comment

The LLVM va_arg instruction is lacking in some instances, so we should only use it as a fallback.

...does Clang also implement this logic manually?

[nikic]

Yes. LLVM types are not rich enough to encode all the necessary ABI requirements to make something like the va_arg instruction work correctly. It would probably be best to remove it entirely.

[erikdesjardins]

The old code used non-inbounds GEP for ZSTs. But it's okay for inbounds to point one-past-the end:

The base pointer has an in bounds address of an allocated object, which means that it points into an allocated object, or to its end.

...which handles the ZST being "outside" of e.g. struct Foo(u64, ()) (where it is at an offset of 8 bytes), so I think using inbounds for ZST fields is okay. Unless ZSTs can be arbitrarily far outside the bounds of a layout? (@oli-obk?)

[oli-obk]

Miri doesn't catch it yet: https://play.rust-lang.org/?version=stable&mode=debug&edition=2021&gist=682be508e94b238472eccdb9e1fdef3b

And it seems to be an open question on whether it is ok or not: rust-lang/unsafe-code-guidelines#93

Tho looks like it's leaning towards it being ok I think?

[erikdesjardins]

Only the initial field offset in that example (x.1) will go through this code, so I think it would be fine...but I'll change it back to non-inbounds for ZSTs anyways, so that the only functional change in this PR is the GEP i8/ptradd change, and since ZST field offsets should never be performance critical, so it doesn't really matter.

[Noratrieb]

@bors try @rust-timer queue

[bors]

⌛ Trying commit 4724cd4 with merge 85edff9...

[bors]

☀️ Try build successful - checks-actions
Build commit: 85edff9 (85edff996ef166238dcfa53fcba0386f7a6cc2f5)

[rust-timer]

Finished benchmarking commit (85edff9): comparison URL.

Overall result: ❌✅ regressions and improvements - ACTION NEEDED

Benchmarking this pull request likely means that it is perf-sensitive, so we're automatically marking it as not fit for rolling up. While you can manually mark this PR as fit for rollup, we strongly recommend not doing so since this PR may lead to changes in compiler perf.

Next Steps: If you can justify the regressions found in this try perf run, please indicate this with @rustbot label: +perf-regression-triaged along with sufficient written justification. If you cannot justify the regressions please fix the regressions and do another perf run. If the next run shows neutral or positive results, the label will be automatically removed.

@bors rollup=never
@rustbot label: -S-waiting-on-perf +perf-regression

Instruction count

This is a highly reliable metric that was used to determine the overall result at the top of this comment.

	mean	range	count
Regressions ❌ (primary)	0.3%	[0.2%, 0.4%]	3
Regressions ❌ (secondary)	0.5%	[0.1%, 1.0%]	15
Improvements ✅ (primary)	-0.6%	[-1.6%, -0.3%]	11
Improvements ✅ (secondary)	-0.6%	[-1.2%, -0.2%]	10
All ❌✅ (primary)	-0.4%	[-1.6%, 0.4%]	14

Max RSS (memory usage)

Results

This is a less reliable metric that may be of interest but was not used to determine the overall result at the top of this comment.

	mean	range	count
Regressions ❌ (primary)	-	-	0
Regressions ❌ (secondary)	3.1%	[3.1%, 3.1%]	1
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-	-	0

Cycles

Results

This is a less reliable metric that may be of interest but was not used to determine the overall result at the top of this comment.

	mean	range	count
Regressions ❌ (primary)	-	-	0
Regressions ❌ (secondary)	2.4%	[2.3%, 2.5%]	2
Improvements ✅ (primary)	-1.0%	[-1.6%, -0.6%]	20
Improvements ✅ (secondary)	-1.0%	[-1.6%, -0.7%]	3
All ❌✅ (primary)	-1.0%	[-1.6%, -0.6%]	20

Binary size

Results

This is a less reliable metric that may be of interest but was not used to determine the overall result at the top of this comment.

	mean	range	count
Regressions ❌ (primary)	0.5%	[0.1%, 1.0%]	12
Regressions ❌ (secondary)	0.4%	[0.0%, 1.0%]	18
Improvements ✅ (primary)	-0.4%	[-0.7%, -0.0%]	9
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	0.1%	[-0.7%, 1.0%]	21

Bootstrap: 649.3s -> 644.43s (-0.75%)
Artifact size: 311.08 MiB -> 311.01 MiB (-0.02%)

[nikic]

We did not encounter any significant issues when we started canonicalizing these upstream, so I think it's fairly safe to do this step now. We could wait until the LLVM 19 release, but it doesn't really seem necessary.

@bors r+ rollup=never

[bors]

📌 Commit 4724cd4 has been approved by nikic

It is now in the queue for this repository.

[RalfJung]

This PR is at the top of the bors queue but not being processed...

[nikic]

@bors r+

[bors]

📌 Commit 4016510 has been approved by nikic

It is now in the queue for this repository.

[rust-timer]

Finished benchmarking commit (70aa0b8): comparison URL.

Overall result: ❌✅ regressions and improvements - ACTION NEEDED

Next Steps: If you can justify the regressions found in this perf run, please indicate this with @rustbot label: +perf-regression-triaged along with sufficient written justification. If you cannot justify the regressions please open an issue or create a new PR that fixes the regressions, add a comment linking to the newly created issue or PR, and then add the perf-regression-triaged label to this PR.

@rustbot label: +perf-regression
cc @rust-lang/wg-compiler-performance

Instruction count

This is a highly reliable metric that was used to determine the overall result at the top of this comment.

	mean	range	count
Regressions ❌ (primary)	0.3%	[0.2%, 0.4%]	3
Regressions ❌ (secondary)	0.5%	[0.2%, 1.0%]	12
Improvements ✅ (primary)	-0.7%	[-1.6%, -0.3%]	14
Improvements ✅ (secondary)	-0.5%	[-1.0%, -0.3%]	12
All ❌✅ (primary)	-0.5%	[-1.6%, 0.4%]	17

Max RSS (memory usage)

Results

This is a less reliable metric that may be of interest but was not used to determine the overall result at the top of this comment.

	mean	range	count
Regressions ❌ (primary)	-	-	0
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-1.6%	[-1.6%, -1.6%]	1
All ❌✅ (primary)	-	-	0

Cycles

Results

This is a less reliable metric that may be of interest but was not used to determine the overall result at the top of this comment.

	mean	range	count
Regressions ❌ (primary)	-	-	0
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-1.2%	[-1.6%, -0.9%]	4
Improvements ✅ (secondary)	-7.3%	[-7.4%, -7.2%]	2
All ❌✅ (primary)	-1.2%	[-1.6%, -0.9%]	4

Binary size

Results

This is a less reliable metric that may be of interest but was not used to determine the overall result at the top of this comment.

	mean	range	count
Regressions ❌ (primary)	0.5%	[0.1%, 1.0%]	12
Regressions ❌ (secondary)	1.0%	[0.0%, 1.3%]	37
Improvements ✅ (primary)	-0.4%	[-0.7%, -0.0%]	9
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	0.1%	[-0.7%, 1.0%]	21

Bootstrap: 649.87s -> 644.02s (-0.90%)
Artifact size: 175.04 MiB -> 174.95 MiB (-0.06%)

[nnethercote]

Are the binary size increases expected?

[erikdesjardins]

It's not unexpected per se, since an intended consequence of this is more GEPs being merged, which can push things under inlining/vectorization thresholds.

It looks like almost all of the size regressions are nearly the exact same magnitude:

js diff

[...document.querySelectorAll('tr.toggle')].forEach(e => {
    const [before, after] = [...e.querySelectorAll('td.numeric')];

    const td = document.createElement('td');
    const diff = after.querySelector('abbr').title - before.querySelector('abbr').title;

    td.textContent = diff > 0 ? '+' + diff : diff;
    
    e.append(td);
});

which suggests that something in the std "runtime" is getting slightly bigger.

I'll diff one of those and see if I can figure out what it is

[erikdesjardins]

The regression is all in backtrace-related code:

_ZN3std12backtrace_rs9symbolize5gimli7Context3new                                                              +288 bytes (17760 -> 18048) +047 insts (3525 -> 3572), +0 funcs ( 1 ->  1)
_ZN9addr2line16ResUnit$LT$R$GT$25find_function_or_location28_$u7b$$u7b$closure$u7d$$u7d$                       +256 bytes (11792 -> 12048) +023 insts (2507 -> 2530), +0 funcs ( 1 ->  1)
_ZN3std12backtrace_rs9symbolize5gimli7resolve                                                                  +240 bytes (14800 -> 15040) +025 insts (2709 -> 2734), +0 funcs ( 1 ->  1)
_ZN4core6option19Option$LT$$RF$T$GT$6cloned                                                                    +176 bytes (    0 ->   176) +056 insts (   0 ->   56), +1 funcs ( 0 ->  1)
_ZN5gimli4read8rnglists20RngListIter$LT$R$GT$4next                                                             +160 bytes ( 3248 ->  3408) +043 insts ( 813 ->  856), +0 funcs ( 1 ->  1)
_ZN9addr2line11render_file                                                                                     +112 bytes (  848 ->   960) +034 insts ( 201 ->  235), +0 funcs ( 1 ->  1)
_ZN93_$LT$gimli..read..line..LineProgramHeader$LT$R$C$Offset$GT$$u20$as$u20$core..clone..Clone$GT$5clone       +096 bytes ( 1120 ->  1216) +033 insts ( 244 ->  277), +0 funcs ( 1 ->  1)
_ZN5gimli4read5dwarf13Unit$LT$R$GT$3new                                                                        +064 bytes (10224 -> 10288) +016 insts (2063 -> 2079), +0 funcs ( 1 ->  1)
_ZN5gimli4read4unit18Attribute$LT$R$GT$5value                                                                  +032 bytes ( 1328 ->  1360) +006 insts ( 382 ->  388), +0 funcs ( 1 ->  1)
_ZN9addr2line30LoopingLookup$LT$T$C$L$C$F$GT$10new_lookup                                                      +032 bytes ( 1536 ->  1568) -011 insts ( 352 ->  341), +0 funcs ( 1 ->  1)
_ZN9addr2line5Lines5parse                                                                                      +016 bytes ( 9920 ->  9936) +012 insts (2144 -> 2156), +0 funcs ( 1 ->  1)
_ZN5gimli4read5dwarf14Dwarf$LT$R$GT$11attr_string                                                              +016 bytes (  592 ->   608) +004 insts ( 165 ->  169), +0 funcs ( 1 ->  1)
_ZN5alloc7raw_vec11finish_grow                                                                                 +016 bytes (  576 ->   592) +003 insts ( 188 ->  191), +0 funcs ( 4 ->  4)
_ZN3std2io5Write18write_all_vectored                                                                           +016 bytes ( 1328 ->  1344) +003 insts ( 347 ->  350), +0 funcs ( 2 ->  2)
_ZN9addr2line16ResUnit$LT$R$GT$18dwarf_and_unit_dwo                                                            +016 bytes ( 1376 ->  1392) +002 insts ( 289 ->  291), +0 funcs ( 1 ->  1)
_ZN11miniz_oxide7inflate4core10decompress                                                                      +016 bytes ( 7888 ->  7904) +001 insts (1913 -> 1914), +0 funcs ( 1 ->  1)
_ZN5alloc7raw_vec19RawVec$LT$T$C$A$GT$7reserve21do_reserve_and_handle                                          -016 bytes (  944 ->   928) +008 insts ( 232 ->  240), +0 funcs ( 5 ->  5)
_ZN5gimli4read6abbrev13Abbreviations6insert                                                                    -016 bytes ( 4864 ->  4848) +006 insts ( 918 ->  924), +0 funcs ( 1 ->  1)
_ZN9addr2line8function17Function$LT$R$GT$14parse_children                                                      -016 bytes ( 5376 ->  5360) +003 insts (1088 -> 1091), +0 funcs ( 1 ->  1)
_ZN3std4path7PathBuf14_set_extension                                                                           -016 bytes (  624 ->   608) +000 insts ( 167 ->  167), +0 funcs ( 1 ->  1)
_ZN5alloc3ffi5c_str7CString19_from_vec_unchecked                                                               -016 bytes (  352 ->   336) +000 insts (  97 ->   97), +0 funcs ( 1 ->  1)
_ZN91_$LT$std..sys_common..backtrace.._print..DisplayBacktrace$u20$as$u20$core..fmt..Display$GT$3fmt           -016 bytes (  592 ->   576) -002 insts ( 123 ->  121), +0 funcs ( 1 ->  1)
_ZN69_$LT$std..sys..pal..unix..stdio..Stderr$u20$as$u20$std..io..Write$GT$14write_vectored                     -016 bytes (   96 ->    80) -004 insts (  29 ->   25), +0 funcs ( 1 ->  1)
_ZN5alloc7raw_vec19RawVec$LT$T$C$A$GT$16reserve_for_push                                                       -032 bytes ( 3296 ->  3264) +027 insts ( 834 ->  861), +0 funcs (17 -> 17)

(Option::cloned doesn't get inlined...in _ZN9addr2line11render_file.)

(Generated with this ad-hoc script. There's probably some existing way to do this though, isn't there...)

It's kind of interesting that the regression is spread across a bunch of different functions but all in backtrace-related code. Although I guess it's just because one function was optimized differently and the butterfly effect cascaded to everything else in its call graph.

Looking at gimli7Context3new, it's very hard to see anything meaningful in the asm diff, even with all constants and registers normalized, hundreds of instructions are moved around. It doesn't look obviously worse, just...perturbed. Same with find_function_or_location.

So I don't really see anything actionable here.

It does make me wonder if we should be compiling backtrace-related crates with opt-level=s though--there's a ton of enormous functions and just scanning through (not necessarily in the changed functions) I see some unrolled loops. Size is probably more of a priority than runtime performance for backtrace code.

[Kobzol]

(Generated with this ad-hoc script. There's probably some existing way to do this though, isn't there...)

$  cargo run --bin collector binary_stats --symbols <rustc>

But it will show slightly different stuff than your script.

[workingjubilee]

It does make me wonder if we should be compiling backtrace-related crates with opt-level=s though--there's a ton of enormous functions and just scanning through (not necessarily in the changed functions) I see some unrolled loops. Size is probably more of a priority than runtime performance for backtrace code.

Runtime performance isn't zero concern for backtrace code, oddly enough (mainly for the "forced capture" case), but backtrace-rs contains algorithmic optimizations which already make the appropriate trades, so yes, if you can get a significantly smaller code size by optimizing in that direction, that would be good.

We can also look at working with the addr2line and gimli maintainers to reduce the code size a bit, if we can identify the heaviest-hitters in those crates.