New MIR Pass: SsaRangePropagation #150309

dianqk · 2025-12-23T14:45:32Z

As an alternative to #150192.

Introduces a new pass that propagates the known ranges of SSA locals.
We can know the ranges of SSA locals at some locations for the following code:

fn foo(a: u32) {
  let b = a < 9;
  if b {
    let c = b; // c is true since b is whitin the range [1, 2)
    let d = a < 8; // d is true since b whitin the range [0, 9)
  }
}

This PR only implements a trivial range: we know one value on switch, assert, and assume.

Kobzol · 2025-12-23T19:46:58Z

@bors try @rust-timer queue

(I want to test if try builds work and this PR seems like a good fit for a perf. run :) )

[EXPERIMENT] New MIR Pass: SsaRangePropagation

rust-bors · 2025-12-23T22:16:34Z

☀️ Try build successful (CI)
Build commit: d9c5f75 (d9c5f757034b39605aac2eeef1b90a892b2c647d, parent: 99ff3fbb86658b427f5dd7daaae8db5626a63c26)

rust-timer · 2025-12-23T23:37:33Z

Finished benchmarking commit (d9c5f75): 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%	[0.2%, 0.8%]	6
Regressions ❌ (secondary)	0.1%	[0.0%, 0.3%]	8
Improvements ✅ (primary)	-0.4%	[-0.9%, -0.2%]	7
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-0.1%	[-0.9%, 0.8%]	13

Max RSS (memory usage)

Results (primary -0.9%, secondary 1.7%)

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

	mean	range	count
Regressions ❌ (primary)	2.0%	[1.9%, 2.1%]	2
Regressions ❌ (secondary)	4.2%	[0.9%, 5.5%]	7
Improvements ✅ (primary)	-3.8%	[-5.4%, -2.2%]	2
Improvements ✅ (secondary)	-1.9%	[-2.9%, -1.5%]	5
All ❌✅ (primary)	-0.9%	[-5.4%, 2.1%]	4

Cycles

Results (primary 1.8%, secondary -1.6%)

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

	mean	range	count
Regressions ❌ (primary)	1.8%	[1.8%, 1.8%]	1
Regressions ❌ (secondary)	2.4%	[2.3%, 2.6%]	2
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-3.2%	[-6.2%, -1.2%]	5
All ❌✅ (primary)	1.8%	[1.8%, 1.8%]	1

Binary size

Results (primary -0.0%, 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)	0.3%	[0.1%, 0.7%]	4
Regressions ❌ (secondary)	0.1%	[0.1%, 0.2%]	2
Improvements ✅ (primary)	-0.1%	[-0.8%, -0.0%]	17
Improvements ✅ (secondary)	-0.1%	[-0.8%, -0.0%]	16
All ❌✅ (primary)	-0.0%	[-0.8%, 0.7%]	21

Bootstrap: 482.582s -> 484.115s (0.32%)
Artifact size: 390.35 MiB -> 390.71 MiB (0.09%)

dianqk · 2026-01-02T14:37:32Z

@bors try parent=99ff3fbb86658b427f5dd7daaae8db5626a63c26 @rust-timer queue

[EXPERIMENT] New MIR Pass: SsaRangePropagation

rust-bors · 2026-01-02T16:55:14Z

☀️ Try build successful (CI)
Build commit: edacb2e (edacb2e2bfe3e402771d6cc818d38a1c1e0aa524, parent: 99ff3fbb86658b427f5dd7daaae8db5626a63c26)

compiler/rustc_mir_transform/src/ssa_range_prop.rs

rust-timer · 2026-01-02T17:51:16Z

Finished benchmarking commit (edacb2e): 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%	[0.2%, 0.7%]	6
Regressions ❌ (secondary)	0.1%	[0.0%, 0.2%]	7
Improvements ✅ (primary)	-0.4%	[-0.9%, -0.1%]	10
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-0.1%	[-0.9%, 0.7%]	16

Max RSS (memory usage)

Results (primary -0.2%, secondary 1.6%)

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

	mean	range	count
Regressions ❌ (primary)	2.3%	[2.0%, 2.6%]	3
Regressions ❌ (secondary)	3.8%	[0.5%, 5.8%]	8
Improvements ✅ (primary)	-3.9%	[-5.4%, -2.5%]	2
Improvements ✅ (secondary)	-1.9%	[-3.0%, -1.5%]	5
All ❌✅ (primary)	-0.2%	[-5.4%, 2.6%]	5

Cycles

Results (primary 1.8%, secondary -1.3%)

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

	mean	range	count
Regressions ❌ (primary)	1.8%	[1.8%, 1.8%]	1
Regressions ❌ (secondary)	2.2%	[2.1%, 2.3%]	3
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-3.4%	[-6.3%, -1.2%]	5
All ❌✅ (primary)	1.8%	[1.8%, 1.8%]	1

Binary size

Results (primary -0.0%, 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)	0.4%	[0.1%, 0.7%]	4
Regressions ❌ (secondary)	0.1%	[0.1%, 0.2%]	2
Improvements ✅ (primary)	-0.1%	[-0.8%, -0.0%]	16
Improvements ✅ (secondary)	-0.1%	[-0.8%, -0.0%]	16
All ❌✅ (primary)	-0.0%	[-0.8%, 0.7%]	20

Bootstrap: 482.582s -> 482.879s (0.06%)
Artifact size: 390.35 MiB -> 390.72 MiB (0.09%)

dianqk · 2026-01-07T13:23:46Z

The pass is fast, and it reduced MIR body definitely. All regressions are from opt, and there are probably more inlining.

rustbot · 2026-01-07T13:23:51Z

Some changes occurred to MIR optimizations

cc @rust-lang/wg-mir-opt

rustbot · 2026-01-07T13:23:53Z

r? @nnethercote

rustbot has assigned @nnethercote.
They will have a look at your PR within the next two weeks and either review your PR or reassign to another reviewer.

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cjgillot

Thanks a lot for coding this! I don't have many comments on the design, this pass is very clear. I mostly have nits/suggestions on the implementation.

View changes since this review

compiler/rustc_mir_transform/src/ssa_range_prop.rs

cjgillot · 2026-01-07T22:46:57Z

compiler/rustc_mir_transform/src/ssa_range_prop.rs

+                if let Err(Some(place)) = self.simplify_operand(cond, location) {
+                    let successor = Location { block: *target, statement_index: 0 };
+                    if location.block != successor.block
+                        && self.unique_predecessors.contains(successor.block)


Why check location.block != successor.block? We know that location.block has at least one predecessor which is reachable from bb0, so which is not itself. So if successor.block has a unique predecessor, it must be different from location.block, mustn't it?
I'm tempted to turn the location.block != successor.block into an assertion, or do you have a test to exhibit this case?

Yup, and I can change to location.dominates(successor, self.dominators) for assert.

compiler/rustc_mir_transform/src/ssa_range_prop.rs

cjgillot · 2026-01-07T22:50:58Z

compiler/rustc_mir_transform/src/ssa_range_prop.rs

+                        }
+                        let successor = Location { block: target, statement_index: 0 };
+                        if location.block != successor.block
+                            && self.unique_predecessors.contains(successor.block)


IIUC, the unique_predecessors check should be equivalent to location.dominates(successor, self.dominators), isn't it?

Consider the test case on_match_2 and on_if_2. They are not equivalent for the SwitchInt terminator.

cjgillot · 2026-01-07T22:52:08Z

compiler/rustc_mir_transform/src/ssa_range_prop.rs

+                    }
+
+                    let otherwise = Location { block: targets.otherwise(), statement_index: 0 };
+                    if place.ty(self.local_decls, self.tcx).ty.is_bool()


Do you mind leaving a FIXME to extend this to other types?

I have left a FIXME. Those FIXMEs can be fixed when it shows beneficial.

compiler/rustc_mir_transform/src/lib.rs

nnethercote · 2026-01-08T04:56:07Z

compiler/rustc_mir_transform/src/ssa_range_prop.rs

+//!   let b = a < 9;
+//!   if b {
+//!     let c = b; // c is true since b is within the range [1, 2)
+//!     let d = a < 8; // d is true since b within the range [0, 9)


I don't understand these comments. b is boolean, how can it be within a numeric range? And where does the [1, 2) come from?

This is the integer representation in MIR, so the full range of boolean is [0, 2).

compiler/rustc_mir_transform/src/ssa_range_prop.rs

nnethercote · 2026-01-08T05:00:53Z

I have done a review pass but @cjgillot knows much more about MIR passes than I do and has already made good comments, so I will hand the review over:

r? @cjgillot

rustbot · 2026-01-08T05:00:56Z

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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 Dec 23, 2025

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Auto merge of #150309 - dianqk:ssa-range, r=<try>

d9c5f75

[EXPERIMENT] New MIR Pass: SsaRangePropagation

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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 Dec 23, 2025

dianqk mentioned this pull request Dec 28, 2025

[EXPERIMENT] GVN: Track known value ranges through assert and switchInt. #150192

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rust-bors bot added a commit that referenced this pull request Jan 2, 2026

Auto merge of #150309 - dianqk:ssa-range, r=<try>

edacb2e

[EXPERIMENT] New MIR Pass: SsaRangePropagation

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scottmcm reviewed Jan 2, 2026

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compiler/rustc_mir_transform/src/ssa_range_prop.rs Outdated Show resolved Hide resolved

rustbot removed the S-waiting-on-perf Status: Waiting on a perf run to be completed. label Jan 2, 2026

dianqk force-pushed the ssa-range branch from 230527f to b5cbc8d Compare January 3, 2026 10:05

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dianqk force-pushed the ssa-range branch from b5cbc8d to 89dea32 Compare January 3, 2026 11:44

dianqk marked this pull request as ready for review January 7, 2026 13:23

rustbot added the S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. label Jan 7, 2026

rustbot removed the S-waiting-on-author Status: This is awaiting some action (such as code changes or more information) from the author. label Jan 7, 2026

rustbot assigned nnethercote Jan 7, 2026

dianqk added the A-mir-opt Area: MIR optimizations label Jan 7, 2026

dianqk changed the title ~~[EXPERIMENT] New MIR Pass: SsaRangePropagation~~ New MIR Pass: SsaRangePropagation Jan 7, 2026

cjgillot self-assigned this Jan 7, 2026

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dianqk force-pushed the ssa-range branch from 89dea32 to 5e757d4 Compare January 8, 2026 14:28

dianqk added 2 commits January 8, 2026 22:31

New MIR Pass: SsaRangePropagation

0051e31

Propagates assume

e9a67c7

dianqk force-pushed the ssa-range branch from 5e757d4 to e9a67c7 Compare January 8, 2026 14:31

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New MIR Pass: SsaRangePropagation #150309

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New MIR Pass: SsaRangePropagation #150309

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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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