Change int-to-ptr transmute lowering back to inttoptr #147541

saethlin · 2025-10-09T23:00:44Z

This is a revert of #121282, but with a regression test to cover the reported miscompile in #147265 that was caused by the way the code here combines with #138759.

saethlin · 2025-10-09T23:13:15Z

This is a mitigation for a miscompilation, and we do not have a good understanding of how widespread the miscompile is. The answer could be "very". I've started a crater run to evaluate this: #138759 (comment) but it will take a significant amount of time for results to come in.

I am not sure it makes sense to wait for that crater run before making a backport decision, considering how long it will take.

scottmcm · 2025-10-09T23:15:15Z

compiler/rustc_codegen_ssa/src/mir/rvalue.rs

        (Int(..) | Float(_), Int(..) | Float(_)) => bx.bitcast(imm, to_backend_ty),
        (Pointer(..), Pointer(..)) => bx.pointercast(imm, to_backend_ty),
-        (Int(..), Pointer(..)) => bx.ptradd(bx.const_null(bx.type_ptr()), imm),
+        (Int(..), Pointer(..)) => bx.inttoptr(imm, to_backend_ty),


Hmm, maybe we could add a BuilderMethods::inttoptr_noprovenance for this, as that would give us a place to have a -Z flag that affects what it does so we could put it back to the better version more easily later?

(Maybe skip the -Z flag in this PR, if we want something that could go to beta, but if it existed we could test both ways in the codegen test.)

Writing the actual implementation here took me about a minute, and I expect it will be similarly near-zero effort to swap it back again. Adding a helper would be more effort.

It just feels like a -Z mutable-noalias situation, where being able to test whether something is still broken without a new build is the reason to do it, not because the actual code change is hard.

A dedicated method would also allow backends which don't miscompile it to keep avoiding the provenance.

rustbot · 2025-10-10T00:17:08Z

Some changes occurred in compiler/rustc_codegen_ssa

cc @WaffleLapkin

rustbot · 2025-10-10T00:17:10Z

r? @lcnr

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saethlin · 2025-10-10T00:17:31Z

r? scottmcm
(you're already here)

saethlin · 2025-10-10T03:46:32Z

@bors try @rust-timer queue

Change int-to-ptr transmute lowering back to inttoptr

rust-bors · 2025-10-10T05:58:20Z

☀️ Try build successful (CI)
Build commit: 9e02838 (9e0283899906af4ae3914a14bee66862c17cec08, parent: 9725c4baacef19345e13f91b27e66e10ef5592ae)

scottmcm

Well, r=me on the code changes if we decide this is the direction we want to go. TBH personally I'd still rather disable the problematic LLVM optimization instead, if we can.

View changes since this review

scottmcm · 2025-10-10T06:05:51Z

tests/codegen-llvm/int-ptr-int-enum-miscompile.rs

+#[no_mangle]
+pub fn mk_result(a: usize) -> Result<u8, *const u8> {
+    // CHECK-LABEL: @mk_result
+    // CHECK-NOT: unreachable


suggestion: it's always good to have positive checks too, just in case the negative thing changes, so maybe

Suggested change

// CHECK-NOT: unreachable

// CHECK-NOT: unreachable

// CHECK: load i8,

// CHECK-NOT: unreachable

rust-timer · 2025-10-10T07:09:03Z

Finished benchmarking commit (9e02838): comparison URL.

Overall result: ❌ regressions - no action needed

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.

@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
Regressions ❌ (secondary)	0.1%	[0.1%, 0.1%]	1
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-	-	0

Max RSS (memory usage)

Results (primary -2.3%, secondary 0.9%)

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)	0.9%	[0.9%, 0.9%]	1
Improvements ✅ (primary)	-2.3%	[-2.3%, -2.3%]	1
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-2.3%	[-2.3%, -2.3%]	1

Cycles

Results (secondary -2.8%)

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)	-	-	0
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-2.8%	[-2.8%, -2.8%]	1
All ❌✅ (primary)	-	-	0

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
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-0.2%	[-0.5%, -0.0%]	16
Improvements ✅ (secondary)	-0.3%	[-0.5%, -0.0%]	56
All ❌✅ (primary)	-0.2%	[-0.5%, -0.0%]	16

Bootstrap: 472.692s -> 472.611s (-0.02%)
Artifact size: 388.03 MiB -> 388.05 MiB (0.00%)

Change int-to-ptr transmute lowering back to inttoptr

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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 Oct 9, 2025

saethlin mentioned this pull request Oct 9, 2025

Rust 1.90 compiler optimization (O2, O3) triggers UB with Box::from_raw() #147265

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scottmcm reviewed Oct 9, 2025

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saethlin marked this pull request as ready for review October 10, 2025 00:17

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Auto merge of #147541 - saethlin:transmute-carefully, r=<try>

9e02838

Change int-to-ptr transmute lowering back to inttoptr

rustbot added the S-waiting-on-perf Status: Waiting on a perf run to be completed. label Oct 10, 2025

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Change int-to-ptr transmute lowering back to inttoptr #147541

Change int-to-ptr transmute lowering back to inttoptr #147541

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Change int-to-ptr transmute lowering back to inttoptr #147541

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rust-timer commented Oct 10, 2025

Overall result: ❌ regressions - no action needed

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