GVN: Use the borrows only if they are always live by dianqk · Pull Request #147886 · rust-lang/rust

dianqk · 2025-10-19T13:38:46Z

Fixes#141313 and #141313 (comment). This PR forbids introducing borrows outside their lifetime.

c8fbb62fixes#130853.

I agree with what #147844 (comment) mentioned:

We can reason with the value behind a reference because it is UB to directly assign to the underlying local while the reference is live. We allow creating new derefs, this means extending the liveness of references, so we are creating UB.

r? cjgillot

rustbot · 2025-10-19T13:38:50Z

Some changes occurred to MIR optimizations

cc @rust-lang/wg-mir-opt

dianqk · 2025-10-19T13:40:04Z

-/// Check that we allow dereferences in the RHS if the LHS is a stable projection.
+// This introduces copy overlapping if dereferencing `_2` or `_4`.
 #[custom_mir(dialect = "runtime")]
 fn stable_projection(_1: (Adt,)) {


@cjgillot I just found this still introduces copy overlapping.

dianqk · 2025-10-19T15:15:37Z

@bors try @rust-timer queue

GVN: Only introduce new derefs if the immutable borrow is always valid.

cjgillot · 2025-10-19T15:25:43Z

I'd have kept introducing Value::Projection(Deref) as exists, but forbidden introducing Deref projections in try_as_place. What do you think?

rust-bors · 2025-10-19T17:30:13Z

☀️ Try build successful (CI)
Build commit: efb5f98 (efb5f98020c7392070aae01c719ed60b78144ff5, parent: c6efb9019b3169fc672248339dbbf13e6a134de3)

rust-timer · 2025-10-19T18:40:20Z

Finished benchmarking commit (efb5f98): comparison URL.

Overall result: ❌✅ regressions and improvements - please read the text below

Benchmarking this pull request means it may be perf-sensitive – we'll automatically label it not fit for rolling up. You can override this, but we strongly advise not to, due to possible changes in compiler perf.

Next Steps: If you can justify the regressions found in this try perf run, please do so in sufficient writing along with @rustbot label: +perf-regression-triaged. If not, please fix the regressions and do another perf run. If its results are neutral or positive, the label will be automatically removed.

@bors rollup=never
@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)	0.8%	[0.8%, 0.8%]	1
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-0.3%	[-0.4%, -0.3%]	3
Improvements ✅ (secondary)	-0.3%	[-0.3%, -0.3%]	1
All ❌✅ (primary)	-0.1%	[-0.4%, 0.8%]	4

Max RSS (memory usage)

Results (primary 4.1%, secondary 0.8%)

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

	mean	range	count
Regressions ❌ (primary)	4.1%	[2.8%, 5.3%]	2
Regressions ❌ (secondary)	0.8%	[0.8%, 0.8%]	1
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	4.1%	[2.8%, 5.3%]	2

Cycles

Results (secondary 3.2%)

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)	3.2%	[3.2%, 3.2%]	1
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-	-	0

Binary size

Results (primary 0.0%, secondary -0.0%)

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)	0.0%	[0.0%, 0.2%]	15
Improvements ✅ (primary)	-0.1%	[-0.5%, -0.0%]	17
Improvements ✅ (secondary)	-0.1%	[-0.1%, -0.0%]	9
All ❌✅ (primary)	0.0%	[-0.5%, 1.0%]	28

Bootstrap: 472.659s -> 474.116s (0.31%)
Artifact size: 390.57 MiB -> 391.13 MiB (0.14%)

dianqk · 2025-10-19T23:49:59Z

I'd have kept introducing Value::Projection(Deref) as exists, but forbidden introducing Deref projections in try_as_place. What do you think?

Oh, I think this is a great insight. IIUC, I can remove invalid_derefs and introduce a borrow-reaching definition that is used in try_as_place. This should allow us to preserve derefs in the RPO A-B(terminator is call)-C.

dianqk · 2025-10-21T12:49:39Z

I'd have kept introducing Value::Projection(Deref) as exists, but forbidden introducing Deref projections in try_as_place. What do you think?
Oh, I think this is a great insight. IIUC, I can remove invalid_derefs and introduce a borrow-reaching definition that is used in try_as_place. This should allow us to preserve derefs in the RPO A-B(terminator is call)-C.

Hmm, I still need to handle this in simplify_binary_inner to check whether *_1 is still valid at the location of *_2. There might be other codes I’ve missed. I can also check the deref in get.

Actually, that's not enough. insert reuses VnIndex when values are equal. For that, I could add a provenance, but I don't see what difference it makes not to introduce new derefs. Maybe do this when we can know if the immutable borrow is in its lifetime scope at the location.

dianqk · 2025-10-29T15:06:38Z

I'd have kept introducing Value::Projection(Deref) as exists, but forbidden introducing Deref projections in try_as_place. What do you think?

I changed it to forbid introducing borrows outside their lifetime. And c8fbb62 should fix #130853.

I have another experimental patch, 6e2ecde, using live variables, but this may introduce miscompile, and I won't add it.

@rustbot review

GVN: Use the borrows only if they are always live

rust-bors · 2025-12-10T16:47:46Z

☀️ Try build successful (CI)
Build commit: 2b9aa76 (2b9aa76acf43f7c466566e82eb9c30d4cd75d243, parent: 377656d3dd3f9c23a9c8713e163f4365a5261a84)

rust-timer · 2025-12-10T17:28:22Z

Finished benchmarking commit (2b9aa76): comparison URL.

Overall result: ❌✅ regressions and improvements - please read the text below

Benchmarking this pull request means it may be perf-sensitive – we'll automatically label it not fit for rolling up. You can override this, but we strongly advise not to, due to possible changes in compiler perf.

Next Steps: If you can justify the regressions found in this try perf run, please do so in sufficient writing along with @rustbot label: +perf-regression-triaged. If not, please fix the regressions and do another perf run. If its results are neutral or positive, the label will be automatically removed.

@bors rollup=never
@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)	0.3%	[0.1%, 0.6%]	8
Regressions ❌ (secondary)	0.5%	[0.2%, 1.4%]	24
Improvements ✅ (primary)	-1.0%	[-2.5%, -0.2%]	3
Improvements ✅ (secondary)	-0.4%	[-2.4%, -0.1%]	8
All ❌✅ (primary)	-0.0%	[-2.5%, 0.6%]	11

Max RSS (memory usage)

Results (primary 2.2%, secondary 1.1%)

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

	mean	range	count
Regressions ❌ (primary)	3.8%	[2.4%, 5.3%]	2
Regressions ❌ (secondary)	1.1%	[1.1%, 1.1%]	1
Improvements ✅ (primary)	-1.2%	[-1.2%, -1.2%]	1
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	2.2%	[-1.2%, 5.3%]	3

Cycles

Results (primary -2.2%, secondary 0.1%)

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)	2.4%	[2.4%, 2.4%]	1
Improvements ✅ (primary)	-2.2%	[-2.2%, -2.2%]	1
Improvements ✅ (secondary)	-2.2%	[-2.2%, -2.2%]	1
All ❌✅ (primary)	-2.2%	[-2.2%, -2.2%]	1

Binary size

Results (primary -0.1%, secondary 0.6%)

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

	mean	range	count
Regressions ❌ (primary)	0.0%	[0.0%, 0.1%]	18
Regressions ❌ (secondary)	1.6%	[0.0%, 15.0%]	10
Improvements ✅ (primary)	-0.3%	[-1.6%, -0.0%]	19
Improvements ✅ (secondary)	-0.2%	[-1.6%, -0.0%]	11
All ❌✅ (primary)	-0.1%	[-1.6%, 0.1%]	37

Bootstrap: 471.42s -> 472.371s (0.20%)
Artifact size: 389.04 MiB -> 389.08 MiB (0.01%)

[PERF] For #147886

rustbot · 2025-12-22T14:25:20Z

This PR was rebased onto a different main commit. Here's a range-diff highlighting what actually changed.

Rebasing is a normal part of keeping PRs up to date, so no action is needed—this note is just to help reviewers.

dianqk · 2025-12-28T14:46:58Z

@rustbot author

dianqk · 2025-12-29T14:14:45Z

I realized that we are propagating the borrows of non-SSA locals. This is why we use borrows outside their lifetime.
I think #150485 is a better resolution.

rust-bors · 2026-01-16T03:22:57Z

☔ The latest upstream changes (presumably #150925) made this pull request unmergeable. Please resolve the merge conflicts.

dianqk · 2026-01-21T13:58:58Z

Superseded by #150485.

@cjgillot

GVN: Only propagate borrows from SSA locals Fixes#141313. This is a more principled fix than #147886. Using a reference that is not a borrowing of an SSA local at a new location may be UB. The PR has two major changes. The first one, when introducing a new dereference at a new location, is that the reference must point to an SSA local or be an immutable argument. `dereference_address` has handled SSA locals. The second one, if we cannot guard to the reference point to an SSA local in `visit_assign`, we have to rewrite the value to opaque. This avoids unifying the following dereferences that also are references: ```rust let b: &T = *a; // ... `a` is allowed to be modified. `c` and `b` have different borrowing lifetime. // Unifying them will extend the lifetime of `b`. let c: &T = *a; ``` See also #130853. This still allows unifying non-reference dereferences: ```rust let a: &T = ...; let b: T = *a; // ... a is NOT allowed to be modified. let c: T = *a; ``` r? @cjgillot

@cjgillot

GVN: Only propagate borrows from SSA locals Fixesrust-lang/rust#141313. This is a more principled fix than rust-lang/rust#147886. Using a reference that is not a borrowing of an SSA local at a new location may be UB. The PR has two major changes. The first one, when introducing a new dereference at a new location, is that the reference must point to an SSA local or be an immutable argument. `dereference_address` has handled SSA locals. The second one, if we cannot guard to the reference point to an SSA local in `visit_assign`, we have to rewrite the value to opaque. This avoids unifying the following dereferences that also are references: ```rust let b: &T = *a; // ... `a` is allowed to be modified. `c` and `b` have different borrowing lifetime. // Unifying them will extend the lifetime of `b`. let c: &T = *a; ``` See also rust-lang/rust#130853. This still allows unifying non-reference dereferences: ```rust let a: &T = ...; let b: T = *a; // ... a is NOT allowed to be modified. let c: T = *a; ``` r? @cjgillot

@cjgillot

GVN: Only propagate borrows from SSA locals Fixesrust-lang/rust#141313. This is a more principled fix than rust-lang/rust#147886. Using a reference that is not a borrowing of an SSA local at a new location may be UB. The PR has two major changes. The first one, when introducing a new dereference at a new location, is that the reference must point to an SSA local or be an immutable argument. `dereference_address` has handled SSA locals. The second one, if we cannot guard to the reference point to an SSA local in `visit_assign`, we have to rewrite the value to opaque. This avoids unifying the following dereferences that also are references: ```rust let b: &T = *a; // ... `a` is allowed to be modified. `c` and `b` have different borrowing lifetime. // Unifying them will extend the lifetime of `b`. let c: &T = *a; ``` See also rust-lang/rust#130853. This still allows unifying non-reference dereferences: ```rust let a: &T = ...; let b: T = *a; // ... a is NOT allowed to be modified. let c: T = *a; ``` r? @cjgillot

@cjgillot

GVN: Only propagate borrows from SSA locals Fixesrust-lang/rust#141313. This is a more principled fix than rust-lang/rust#147886. Using a reference that is not a borrowing of an SSA local at a new location may be UB. The PR has two major changes. The first one, when introducing a new dereference at a new location, is that the reference must point to an SSA local or be an immutable argument. `dereference_address` has handled SSA locals. The second one, if we cannot guard to the reference point to an SSA local in `visit_assign`, we have to rewrite the value to opaque. This avoids unifying the following dereferences that also are references: ```rust let b: &T = *a; // ... `a` is allowed to be modified. `c` and `b` have different borrowing lifetime. // Unifying them will extend the lifetime of `b`. let c: &T = *a; ``` See also rust-lang/rust#130853. This still allows unifying non-reference dereferences: ```rust let a: &T = ...; let b: T = *a; // ... a is NOT allowed to be modified. let c: T = *a; ``` r? @cjgillot

rustbot added S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. T-compiler Relevant to the compiler team, which will review and decide on the PR/issue. labels Oct 19, 2025

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dianqk commented Oct 19, 2025
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GVN: Bail out if the address cannot be dereferenced. #147844
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GVN: Only introduce new derefs if the immutable borrow is always valid.

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dianqk changed the title ~~GVN: Only introduce new derefs if the immutable borrow is always valid.~~GVN: Use the borrows only if they are always liveOct 29, 2025

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GVN: Use the borrows only if they are always live

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[PERF] For #147886 #150222
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Auto merge of#150222- dianqk:perf/gvn-new-deref-arg, r=<try>
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[PERF] For #147886

dianqk added 5 commits December 22, 2025 22:15

Add miscompiled test cases for GVN
9df1ecb

GVN: Use the borrows only if they are always live
d4e4113

GVN: Remove unused invalidate_derefs
84afac0

GVN: Avoid merging borrows from the dereferenced value
430f430

GVN: Borrows of a ssa local are always live
3665eaa

dianqk force-pushed the gvn-new-deref-arg branch from d6f8232 to 3665eaaCompare December 22, 2025 14:25

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GVN: Avoid merging borrows from the dereferenced value #150270
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GVN: Only propagate borrows from SSA locals #150485
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Overall result: ❌✅ regressions and improvements - please read the text below

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Overall result: ❌✅ regressions and improvements - please read the text below

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