GVN: Avoid analyzing references that come from unions or MaybeDangling by saethlin · Pull Request #163962 · rust-lang/rust

saethlin · 2026-10-08T03:16:14Z

This is a possible fix for #163825.

There is a small perf impact, but it seems more like churn than a regression.

Unfortunately I think this change breaks EarlyOtherwiseBranch, because it relies on all of the discriminant reads being from the same Place, and without GVN looking through the no_retag copies produced by EraseDerefTemps they are all from different Places that alias.

saethlin · 2026-10-08T03:18:22Z

@bors try @rust-timer queue

GVN: Only analyze dereferences if the reference is a function argument

rust-bors · 2026-10-08T04:20:52Z

☀️ Try build successful (CI)
Build commit: fade20e (fade20e734a2a69906122ef3506d782e27cbaedd)
Base parent: 33ffc9b (33ffc9b4d83afef2445b585d379d7158cbbd2315)

rust-timer · 2026-10-08T05:14:50Z

Finished benchmarking commit (fade20e): comparison URL.

Overall result: ❌✅ regressions and improvements - please read:

Benchmarking means the PR may be perf-sensitive. It's automatically marked not fit for rolling up. Overriding is possible but disadvised: it risks changing compiler perf.

Next, please: If you can, justify the regressions found in this try perf run in writing along with @rustbot label: +perf-regression-triaged. If not, fix the regressions and do another perf run. Neutral or positive results will clear the label automatically.

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

Instruction count

Our most reliable metric. Used to determine the overall result above. However, even this metric can be noisy.

	mean	range	count
Regressions ❌ (primary)	0.2%	[0.1%, 0.3%]	23
Regressions ❌ (secondary)	0.5%	[0.1%, 1.2%]	33
Improvements ✅ (primary)	-0.4%	[-0.6%, -0.1%]	8
Improvements ✅ (secondary)	-0.4%	[-0.7%, -0.2%]	38
All ❌✅ (primary)	0.0%	[-0.6%, 0.3%]	31

Max RSS (memory usage)

Results (primary -0.8%, secondary -4.8%)

A less reliable metric. May be of interest, but not used to determine the overall result above.

	mean	range	count
Regressions ❌ (primary)	4.4%	[2.6%, 6.3%]	2
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-2.9%	[-7.0%, -0.5%]	5
Improvements ✅ (secondary)	-4.8%	[-6.2%, -3.5%]	2
All ❌✅ (primary)	-0.8%	[-7.0%, 6.3%]	7

Cycles

Results (primary -2.0%, secondary 3.4%)

A less reliable metric. May be of interest, but not used to determine the overall result above.

	mean	range	count
Regressions ❌ (primary)	-	-	0
Regressions ❌ (secondary)	3.4%	[3.1%, 3.6%]	2
Improvements ✅ (primary)	-2.0%	[-2.0%, -2.0%]	1
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-2.0%	[-2.0%, -2.0%]	1

Binary size

Results (primary -0.1%, secondary -2.1%)

A less reliable metric. May be of interest, but not used to determine the overall result above.

	mean	range	count
Regressions ❌ (primary)	0.2%	[0.0%, 1.0%]	11
Regressions ❌ (secondary)	0.1%	[0.1%, 0.1%]	1
Improvements ✅ (primary)	-0.4%	[-1.0%, -0.0%]	14
Improvements ✅ (secondary)	-2.6%	[-11.5%, -0.1%]	5
All ❌✅ (primary)	-0.1%	[-1.0%, 1.0%]	25

Bootstrap: 490.731s -> 491.399s (0.14%)
Artifact size: 408.59 MiB -> 408.68 MiB (0.02%)

saethlin · 2026-10-08T17:15:30Z

@bors try @rust-timer queue

GVN: Only analyze dereferences if the reference is a function argument

rust-bors · 2026-10-08T18:22:03Z

☀️ Try build successful (CI)
Build commit: 007547e (007547e70502802536bd001ed0889bca778e81d8)
Base parent: 65efa19 (65efa194201c67ba29b7384255a534d61c2aa3cb)

rust-timer · 2026-10-08T19:29:22Z

Finished benchmarking commit (007547e): comparison URL.

Overall result: ❌✅ regressions and improvements - please read:

Benchmarking means the PR may be perf-sensitive. It's automatically marked not fit for rolling up. Overriding is possible but disadvised: it risks changing compiler perf.

Next, please: If you can, justify the regressions found in this try perf run in writing along with @rustbot label: +perf-regression-triaged. If not, fix the regressions and do another perf run. Neutral or positive results will clear the label automatically.

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

Instruction count

Our most reliable metric. Used to determine the overall result above. However, even this metric can be noisy.

	mean	range	count
Regressions ❌ (primary)	0.7%	[0.7%, 0.7%]	1
Regressions ❌ (secondary)	0.6%	[0.2%, 1.0%]	7
Improvements ✅ (primary)	-0.3%	[-0.3%, -0.2%]	3
Improvements ✅ (secondary)	-0.2%	[-0.3%, -0.2%]	3
All ❌✅ (primary)	-0.0%	[-0.3%, 0.7%]	4

Max RSS (memory usage)

Results (primary 0.4%, secondary 2.1%)

A less reliable metric. May be of interest, but not used to determine the overall result above.

	mean	range	count
Regressions ❌ (primary)	3.5%	[0.8%, 6.6%]	3
Regressions ❌ (secondary)	2.1%	[2.0%, 2.2%]	2
Improvements ✅ (primary)	-2.8%	[-3.2%, -2.3%]	3
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	0.4%	[-3.2%, 6.6%]	6

Cycles

Results (primary 2.4%, secondary 14.0%)

A less reliable metric. May be of interest, but not used to determine the overall result above.

	mean	range	count
Regressions ❌ (primary)	2.4%	[2.0%, 2.8%]	2
Regressions ❌ (secondary)	22.3%	[20.2%, 24.5%]	2
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-2.8%	[-2.8%, -2.8%]	1
All ❌✅ (primary)	2.4%	[2.0%, 2.8%]	2

Binary size

Results (primary -0.2%, secondary -0.3%)

A less reliable metric. May be of interest, but not used to determine the overall result above.

	mean	range	count
Regressions ❌ (primary)	0.5%	[0.3%, 0.8%]	4
Regressions ❌ (secondary)	0.1%	[0.1%, 0.1%]	3
Improvements ✅ (primary)	-0.3%	[-0.8%, -0.0%]	14
Improvements ✅ (secondary)	-0.8%	[-1.7%, -0.0%]	2
All ❌✅ (primary)	-0.2%	[-0.8%, 0.8%]	18

Bootstrap: 485.904s -> 487.426s (0.31%)
Artifact size: 406.44 MiB -> 406.51 MiB (0.02%)

saethlin · 2026-10-08T19:33:27Z

That seems in the space of reasonable to me

saethlin · 2026-10-08T20:21:32Z

@bors try @rust-timer queue

dianqk · 2026-10-10T14:20:39Z

All of these scenarios: unions, MaybeDangling, and pointer-to-reference transmutes are ways to produce the same provenance structure that defies the reasoning that GVN is trying to use. I suppose we could also have GVN exclude references created by transmuting a pointer?
Which is true, the problem is that GVN is trying to unify r1 with r2 and their lifetimes do not overlap.

If we treat the pointer-to-reference transmute as an opaque value, we should still be able to unify the following dereferences:

let r1: &u8 = std::mem::transmute(p); // an opaque value.
let a = *r1;
let b = *r1; // let b = a;

dianqk

Should we have both no-retag and retag tests? Both of them are different MIRs.

View changes since this review

dianqk · 2026-10-10T14:24:37Z

@@ -0,0 +1,65 @@
+//@ test-mir-pass: GVN
+//@ compile-flags: -O -Zinline-mir


Is manuallydrop testing with the compile flags? I cannot see _2.0.

Suggested change

//@ compile-flags: -O -Zinline-mir

//@ compile-flags: -Zmir-enable-passes=+ReferencePropagation -Zinline-mir -Zub-checks=false

I believe -O is not working.

dianqk · 2026-10-10T14:26:13Z

        let projection_ty = place_ty.projection_ty(self.tcx, proj);
+        // References that we load from a union field or MaybeDangling cannot be unified.
+        // See https://github.com/rust-lang/rust/issues/163825
+        if projection_ty.ty.is_ref()


Suggested change

if projection_ty.ty.is_ref()

if self.ty_may_have_ref(projection_ty.ty)

;p

use std::mem::ManuallyDrop; union U { r: (&'static u8,), p: *mut u8, } #[inline(never)] pub fn union_field_ref_tuple(p: *mut u8) -> (u8, u8) { let u = U { p }; unsafe { let a = u.r; let a = a.0; let a = *a; *p = 2; let b = u.r; let b = b.0; let b = *b; (a, b) } } #[inline(never)] pub fn manuallydrop_ref_tuple(p: *mut u8) -> (u8, u8) { let m: ManuallyDrop<(&u8,)> = unsafe { std::mem::transmute(p) }; unsafe { let a = *m; let a = a.0; let a = *a; *p = 2; let b = *m; let b = b.0; let b = *b; (a, b) } }

I have a new approach that might be more precise, but it could take a long time to land, so it shouldn't block this PR.

In fact, we can unify (*x).1:

pub fn indirect_deref_1(x: &(&i32,i32), out: fn(i32, i32), clobber: fn()) { out(*(*x).0, (*x).1); clobber(); out(*(*x).0, (*x).1); }

saethlin · 2026-10-18T04:02:08Z

@bors try @rust-timer queue

GVN: Avoid analyzing references that come from unions or MaybeDangling

rust-bors · 2026-10-18T05:02:17Z

☀️ Try build successful (CI)
Build commit: 37680c3 (37680c364850270f210f2089c7506ecb9953cc5f)
Base parent: 02fb0a1 (02fb0a16311c3411fa373627706da085cd8fbd3c)

rust-timer · 2026-10-18T05:48:16Z

Finished benchmarking commit (37680c3): comparison URL.

Overall result: ❌✅ regressions and improvements - please read:

Benchmarking means the PR may be perf-sensitive. It's automatically marked not fit for rolling up. Overriding is possible but disadvised: it risks changing compiler perf.

Next, please: If you can, justify the regressions found in this try perf run in writing along with @rustbot label: +perf-regression-triaged. If not, fix the regressions and do another perf run. Neutral or positive results will clear the label automatically.

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

Instruction count

Our most reliable metric. Used to determine the overall result above. However, even this metric can be noisy.

	mean	range	count
Regressions ❌ (primary)	2.0%	[0.3%, 8.5%]	13
Regressions ❌ (secondary)	1.8%	[0.3%, 6.1%]	10
Improvements ✅ (primary)	-0.3%	[-1.0%, -0.2%]	9
Improvements ✅ (secondary)	-0.6%	[-1.1%, -0.2%]	18
All ❌✅ (primary)	1.1%	[-1.0%, 8.5%]	22

Max RSS (memory usage)

Results (primary 3.0%, secondary -4.5%)

A less reliable metric. May be of interest, but not used to determine the overall result above.

	mean	range	count
Regressions ❌ (primary)	3.0%	[1.8%, 4.2%]	4
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-4.5%	[-5.3%, -3.7%]	2
All ❌✅ (primary)	3.0%	[1.8%, 4.2%]	4

Cycles

Results (primary 4.5%, secondary 2.3%)

A less reliable metric. May be of interest, but not used to determine the overall result above.

	mean	range	count
Regressions ❌ (primary)	4.5%	[4.0%, 5.1%]	2
Regressions ❌ (secondary)	3.7%	[2.1%, 5.8%]	4
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-3.4%	[-3.4%, -3.4%]	1
All ❌✅ (primary)	4.5%	[4.0%, 5.1%]	2

Binary size

Results (primary 0.7%, secondary 1.2%)

A less reliable metric. May be of interest, but not used to determine the overall result above.

	mean	range	count
Regressions ❌ (primary)	5.5%	[5.5%, 5.5%]	1
Regressions ❌ (secondary)	3.0%	[0.5%, 5.5%]	2
Improvements ✅ (primary)	-0.5%	[-1.1%, -0.1%]	4
Improvements ✅ (secondary)	-0.1%	[-0.1%, -0.0%]	3
All ❌✅ (primary)	0.7%	[-1.1%, 5.5%]	5

Bootstrap: 489.75s -> 490.638s (0.18%)
Artifact size: 409.00 MiB -> 409.16 MiB (0.04%)

saethlin · 2026-10-18T20:19:42Z

@bors try @rust-timer queue

GVN: Avoid analyzing references that come from unions or MaybeDangling

rust-bors · 2026-10-18T21:19:19Z

☀️ Try build successful (CI)
Build commit: c21e106 (c21e106dd0a62eca92d295122a1042bd2a0e28d0)
Base parent: 15c1044 (15c10447a46529b68e38c7f138aeb95240e4d7d6)

rust-timer · 2026-10-18T22:05:27Z

Finished benchmarking commit (c21e106): comparison URL.

Overall result: ❌✅ regressions and improvements - please read:

Benchmarking means the PR may be perf-sensitive. It's automatically marked not fit for rolling up. Overriding is possible but disadvised: it risks changing compiler perf.

Next, please: If you can, justify the regressions found in this try perf run in writing along with @rustbot label: +perf-regression-triaged. If not, fix the regressions and do another perf run. Neutral or positive results will clear the label automatically.

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

Instruction count

Our most reliable metric. Used to determine the overall result above. However, even this metric can be noisy.

	mean	range	count
Regressions ❌ (primary)	0.4%	[0.4%, 0.4%]	1
Regressions ❌ (secondary)	0.7%	[0.5%, 1.1%]	3
Improvements ✅ (primary)	-0.3%	[-1.2%, -0.1%]	14
Improvements ✅ (secondary)	-0.6%	[-1.1%, -0.2%]	19
All ❌✅ (primary)	-0.2%	[-1.2%, 0.4%]	15

Max RSS (memory usage)

Results (primary 2.6%, secondary 1.5%)

A less reliable metric. May be of interest, but not used to determine the overall result above.

	mean	range	count
Regressions ❌ (primary)	2.6%	[1.3%, 3.1%]	5
Regressions ❌ (secondary)	3.8%	[3.4%, 4.1%]	2
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-3.0%	[-3.0%, -3.0%]	1
All ❌✅ (primary)	2.6%	[1.3%, 3.1%]	5

Cycles

This perf run didn't have relevant results for this metric.

Binary size

Results (primary 0.7%, secondary 1.2%)

A less reliable metric. May be of interest, but not used to determine the overall result above.

	mean	range	count
Regressions ❌ (primary)	5.5%	[5.5%, 5.5%]	1
Regressions ❌ (secondary)	3.0%	[0.5%, 5.5%]	2
Improvements ✅ (primary)	-0.5%	[-1.1%, -0.1%]	4
Improvements ✅ (secondary)	-0.1%	[-0.1%, -0.0%]	3
All ❌✅ (primary)	0.7%	[-1.1%, 5.5%]	5

Bootstrap: 490.17s -> 488.949s (-0.25%)
Artifact size: 408.94 MiB -> 409.01 MiB (0.02%)

rust-bors · 2026-10-31T16:44:37Z

☔ The latest upstream changes (presumably #165234) made this pull request unmergeable. Please resolve the merge conflicts by rebasing.

rustbot added S-waiting-on-author Status: This is awaiting some action (such as code changes or more information) from the author. T-compiler Relevant to the compiler team, which will review and decide on the PR/issue. labels Oct 8, 2026

saethlin mentioned this pull request Oct 8, 2026

GVN versus unions with reference fields #163825

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Auto merge of #163962 - saethlin:gvn-only-ref-arguments, r=<try>

fade20e

GVN: Only analyze dereferences if the reference is a function argument

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Auto merge of #163962 - saethlin:gvn-only-ref-arguments, r=<try>

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GVN: Only analyze dereferences if the reference is a function argument

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saethlin changed the title ~~GVN: Only analyze dereferences if the reference is a function argument~~ GVN: Try to avoid analyzing references that have not been retagged Oct 8, 2026

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dianqk reviewed Oct 10, 2026

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saethlin added 3 commits October 17, 2026 17:11

Add test

f889d36

GVN: Do not analyze references that come from "transmutes"

c2b83d0

Tweak criteria for ty_may_have_ref

fe10c74

saethlin force-pushed the gvn-only-ref-arguments branch from 3906272 to fe10c74 Compare October 17, 2026 21:59

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Improve recursion of ty_may_have_ref

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Auto merge of #163962 - saethlin:gvn-only-ref-arguments, r=<try>

37680c3

GVN: Avoid analyzing references that come from unions or MaybeDangling

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rustbot added perf-regression Performance regression. and removed S-waiting-on-perf Status: Waiting on a perf run to be completed. labels Oct 18, 2026

Put some optimizations around ty_may_have_ref

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Auto merge of #163962 - saethlin:gvn-only-ref-arguments, r=<try>

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GVN: Avoid analyzing references that come from unions or MaybeDangling

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rustbot removed the S-waiting-on-perf Status: Waiting on a perf run to be completed. label Oct 18, 2026

Account for Box and normalization failures

5c5105e

		@@ -0,0 +1,65 @@
		//@ test-mir-pass: GVN
		//@ compile-flags: -O -Zinline-mir

	//@ compile-flags: -O -Zinline-mir
	//@ compile-flags: -Zmir-enable-passes=+ReferencePropagation -Zinline-mir -Zub-checks=false

	if projection_ty.ty.is_ref()
	if self.ty_may_have_ref(projection_ty.ty)

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Overall result: ❌✅ regressions and improvements - please read:

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Overall result: ❌✅ regressions and improvements - please read:

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Overall result: ❌✅ regressions and improvements - please read:

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Overall result: ❌✅ regressions and improvements - please read:

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