Correct outdated object size limit #127546

workingjubilee · 2024-07-10T01:46:21Z

The comment here about 48 bit addresses being enough was written in 2016 but was made incorrect in 2019 by 5-level paging, and then persisted for another 5 years before being noticed and corrected.

The bolding of the "exclusive" part is merely to call attention to something I missed when reading it and doublechecking the math.

r? @saethlin

try-job: i686-msvc
try-job: test-various

workingjubilee · 2024-07-10T03:40:04Z

tests/ui/limits/huge-static.stderr

   |
 LL | static MY_TOO_BIG_ARRAY_2: [u8; HUGE_SIZE] = [0x00; HUGE_SIZE];
-   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ values of the type `[u8; 2305843009213693951]` are too big for the current architecture
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ values of the type `[u8; 2305843009213693952]` are too big for the current architecture


yes, it was literally 1 byte too small to be oversized (at least, according to the relevant check).

workingjubilee · 2024-07-10T03:48:32Z

I don't know how to fix the gcc errors?

workingjubilee · 2024-07-10T03:49:42Z

wait these are the 32-bit errors... good, actually, I was wondering.

The comment here about 48 bit addresses being enough was written in 2016 but was made incorrect in 2019 by 5-level paging, and then persisted for another 5 years before being noticed and corrected.

These tests depend on the internal logic of rustc regarding handling very large objects. Fix them to reflect rustc_abi::obj_size_bound diffs.

workingjubilee · 2024-09-20T01:20:07Z

@bors try

…, r=<try> Correct outdated object size limit The comment here about 48 bit addresses being enough was written in 2016 but was made incorrect in 2019 by 5-level paging, and then persisted for another 5 years before being noticed and corrected. The bolding of the "exclusive" part is merely to call attention to something I missed when reading it and doublechecking the math. try-job: i686-msvc try-job: test-various

bors · 2024-09-20T01:21:17Z

⌛ Trying commit 8431855 with merge cfa946e...

bors · 2024-09-20T03:15:39Z

☀️ Try build successful - checks-actions
Build commit: cfa946e (cfa946ee32abd52e574ce20c91b3890facc7a8f7)

saethlin · 2024-09-20T03:39:04Z

Having read the error text here a zillion times, I now want it to say something like "are too big for the target architecture (x86_64)" because "current" could mean the host. If you agree, you can do that here. Otherwise, r=me.

workingjubilee · 2024-09-20T07:49:06Z

It is done. Adding the architecture as a param is a cool idea, but requires unifying the source of this error message. There are at least 4 separate instances of it, and the moment I attempted such a unification, a herd of yaks appeared, so I will not be doing that.

rustbot · 2024-09-20T15:42:28Z

The Miri subtree was changed

cc @rust-lang/miri

workingjubilee · 2024-09-20T15:48:11Z

@rustbot ready

saethlin · 2024-09-20T16:23:54Z

@bors r+ rollup=iffy

workingjubilee · 2024-09-20T17:03:58Z

@bors r=saethlin

bors · 2024-09-20T17:04:01Z

📌 Commit d93d2f1 has been approved by saethlin

It is now in the queue for this repository.

…, r=saethlin Correct outdated object size limit The comment here about 48 bit addresses being enough was written in 2016 but was made incorrect in 2019 by 5-level paging, and then persisted for another 5 years before being noticed and corrected. The bolding of the "exclusive" part is merely to call attention to something I missed when reading it and doublechecking the math. try-job: i686-msvc try-job: test-various

bors · 2024-09-21T01:16:50Z

⌛ Testing commit d93d2f1 with merge 9bbc9f4...

bors · 2024-09-21T01:37:40Z

💔 Test failed - checks-actions

compiler/rustc_abi/src/lib.rs

RalfJung · 2024-09-21T07:27:20Z

compiler/rustc_abi/src/lib.rs

@@ -337,23 +337,21 @@ impl TargetDataLayout {
        Ok(dl)
    }

-    /// Returns exclusive upper bound on object size.
+    /// Returns **exclusive** upper bound on object size.
    ///
    /// The theoretical maximum object size is defined as the maximum positive `isize` value.


I am not entirely sure what is even meant by "object". In general we are saying that "allocated objects" are limited by isize::MAX, and we do not publicly document anything else even for 64bit targets. So we do currently allow people to create heap allocations larger than obj_size_bound, and we rely on LLVM supporting that. It is only Rust types that can't be any bigger, and therefore static and let-bound variables and Box.

So either we should clarify the comment to say that this does not apply to all heap allocations, or we need to fix our docs.

@nikic will LLVM miscompile things if a heap allocation gets bigger than 2^61 bytes because that overflows u64 when counting in bits? Or does the 2^64 bits limit only apply to LLVM "typed objects"? Or is this too theoretical because nothing can be that big anyway? ;)

I think it would only be a problem if it's a typed allocation/access, but I'm not particularly confident in that :)

anyone got a 64-bit address space computer lying around to find out?

I am not entirely sure what is even meant by "object".

Honestly I'm also not completely confident... I think allocation is what is meant?

I was pretty much just rolling with it because "we have a slightly wibbly definition of 'object' deep in the codegen backend" feels kinda low on our list of concerns. Relatively speaking.

workingjubilee · 2024-09-21T14:53:08Z

I have no idea what's up with the failing test and it looks like nobody else knows eithe.r

Co-authored-by: Ralf Jung <[email protected]>

workingjubilee · 2024-09-21T15:27:02Z

I'm going to give this one retry before I sit down with that test and have a nice long chat with it.

@bors retry

workingjubilee · 2024-09-21T15:27:18Z

hm wait that might not be the right commit. @bors r=saethlin

bors · 2024-09-21T15:27:21Z

📌 Commit cf78f26 has been approved by saethlin

It is now in the queue for this repository.

bors · 2024-09-21T16:20:14Z

⌛ Testing commit cf78f26 with merge 1d68e6d...

bors · 2024-09-21T18:45:45Z

☀️ Test successful - checks-actions
Approved by: saethlin
Pushing 1d68e6d to master...

rust-timer · 2024-09-21T20:11:05Z

Finished benchmarking commit (1d68e6d): comparison URL.

Overall result: no relevant changes - no action needed

@rustbot label: -perf-regression

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	mean	range	count
Regressions ❌ (primary)	-	-	0
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-2.6%	[-2.6%, -2.5%]	2
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-2.6%	[-2.6%, -2.5%]	2

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	mean	range	count
Regressions ❌ (primary)	0.8%	[0.4%, 2.1%]	5
Regressions ❌ (secondary)	2.5%	[2.5%, 2.5%]	1
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	0.8%	[0.4%, 2.1%]	5

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Bootstrap: 767.545s -> 769.527s (0.26%)
Artifact size: 341.42 MiB -> 341.45 MiB (0.01%)

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rustbot added A-testsuite Area: The testsuite used to check the correctness of rustc T-bootstrap Relevant to the bootstrap subteam: Rust's build system (x.py and src/bootstrap) T-compiler Relevant to the compiler team, which will review and decide on the PR/issue. labels Jul 10, 2024

workingjubilee marked this pull request as ready for review July 10, 2024 03:16

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workingjubilee added 4 commits September 19, 2024 16:23

Correct outdated object size limit

42dbf29

The comment here about 48 bit addresses being enough was written in 2016 but was made incorrect in 2019 by 5-level paging, and then persisted for another 5 years before being noticed and corrected.

bless issue-56762.rs as huge-static.rs

d6383b4

Keep object-size-dependent tests failing

f4cb6ef

These tests depend on the internal logic of rustc regarding handling very large objects. Fix them to reflect rustc_abi::obj_size_bound diffs.

Bless rustc_abi::obj_size_bound tests

65f132f

workingjubilee force-pushed the 5-level-paging-exists branch 2 times, most recently from 98c3e20 to f04e444 Compare September 19, 2024 23:47

saethlin self-assigned this Sep 19, 2024

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workingjubilee force-pushed the 5-level-paging-exists branch from f04e444 to 2b99a34 Compare September 19, 2024 23:55

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rustbot added the S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. label Sep 20, 2024

miri: An error message got changed

d93d2f1

workingjubilee force-pushed the 5-level-paging-exists branch from a3e661f to d93d2f1 Compare September 20, 2024 17:03

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bors added S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. and removed S-waiting-on-bors Status: Waiting on bors to run and complete tests. Bors will change the label on completion. labels Sep 21, 2024

RalfJung reviewed Sep 21, 2024

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nikic mentioned this pull request Sep 21, 2024

issue-111508-vec-tryinto-array.rs fails spuriously on x86_64-gnu-llvm-19 #130656

Closed

compiler: Precisely name units of object size

cf78f26

Co-authored-by: Ralf Jung <[email protected]>

bors added S-waiting-on-bors Status: Waiting on bors to run and complete tests. Bors will change the label on completion. and removed S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. labels Sep 21, 2024

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bors merged commit 1d68e6d into rust-lang:master Sep 21, 2024
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workingjubilee deleted the 5-level-paging-exists branch September 21, 2024 21:09

matthiaskrgr mentioned this pull request Sep 22, 2024

ICE: interpret const eval failure of Unevaluated(UnevaluatedConst #130687

Closed

Correct outdated object size limit #127546

Correct outdated object size limit #127546

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