GVN: transmute adts to their fields if a field projection is immediately transmuted anyway by oli-obk · Pull Request #153085 · rust-lang/rust

oli-obk · 2026-02-25T12:45:30Z

follow-up to #152702

commits best reviewed individually.

Basically does

-         _11 = copy ((_10.0: std::ptr::Unique<()>).0: std::ptr::NonNull<()>) as *const () (Transmute);
+         _11 = copy _10 as *const () (Transmute);

in various forms, including going from an Option directly to a value in its Some variant if the value got niche optimized into the Option

r? @scottmcm
cc @cjgillot

rustbot · 2026-02-25T12:45:33Z

Some changes occurred to MIR optimizations

cc @rust-lang/wg-mir-opt

oli-obk · 2026-02-25T13:26:16Z

@bors try @rust-timer queue

GVN: transmute adts to their fields if a field projection is immediately transmuted anyway

rust-bors · 2026-02-25T15:37:24Z

☀️ Try build successful (CI)
Build commit: de634ba (de634ba015f8ccb17717cd0fbd048e381b7e297c, parent: 58745ca3b05afa82bd5720dd05f3854e674700e0)

rust-timer · 2026-02-25T20:05:57Z

Finished benchmarking commit (de634ba): 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)	0.6%	[0.4%, 0.8%]	2
Regressions ❌ (secondary)	0.3%	[0.1%, 0.4%]	3
Improvements ✅ (primary)	-0.3%	[-0.8%, -0.2%]	8
Improvements ✅ (secondary)	-0.3%	[-0.3%, -0.2%]	6
All ❌✅ (primary)	-0.1%	[-0.8%, 0.8%]	10

Max RSS (memory usage)

Results (primary 5.3%, secondary 6.8%)

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

	mean	range	count
Regressions ❌ (primary)	5.3%	[2.1%, 13.0%]	4
Regressions ❌ (secondary)	6.8%	[5.4%, 8.2%]	2
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	5.3%	[2.1%, 13.0%]	4

Cycles

This benchmark run did not return any relevant results for this metric.

Binary size

Results (primary -0.1%, 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%	[0.0%, 0.5%]	5
Regressions ❌ (secondary)	0.3%	[0.0%, 0.5%]	8
Improvements ✅ (primary)	-0.5%	[-1.2%, -0.0%]	4
Improvements ✅ (secondary)	-0.3%	[-0.7%, -0.0%]	5
All ❌✅ (primary)	-0.1%	[-1.2%, 0.5%]	9

Bootstrap: 478.569s -> 481.658s (0.65%)
Artifact size: 395.75 MiB -> 397.78 MiB (0.51%)

scottmcm · 2026-02-26T22:58:15Z

Very cool! I like that we can do this in mir so it can handle things like Box<T> -> *mut T directly that we couldn't do in the library because of needing to work with non-ZST Allocators. I skimmed through some of the test updates and those all seem fine. Plus lovely to see check & debug improvements from a pass that doesn't even run in debug :)

I really don't feel confident that I know the nuances of GVN enough to comfortable reviewing this, though, so
r? cjgillot

dianqk · 2026-02-27T09:19:37Z

+        StorageDead(_7);
+        StorageLive(_9);
+        _9 = copy _8 as *mut u8 (Transmute);
+        _10 = alloc::alloc::__rust_dealloc(move _9, move _5, move _6) -> [return: bb3, unwind unreachable];


regression?

Hmm, it means we didn't know the NonNull-ness before, and we don't (currently) elide transmutes like this when the middle type is more specific than the source or destination in order to preserve that extra niche information for the backend.

See discussion in #152702 (comment) ; I don't think doing something about it needs to be this PR.

Opened #153251 to get rid of the extra step here.

oli-obk · 2026-03-05T11:10:07Z

Plus lovely to see check & debug improvements from a pass that doesn't even run in debug :)

it is run on libstd/core, where we have this pattern a lot :D

oli-obk · 2026-03-05T12:28:20Z

@bors try @rust-timer queue

GVN: transmute adts to their fields if a field projection is immediately transmuted anyway

rust-bors · 2026-03-05T14:38:33Z

☀️ Try build successful (CI)
Build commit: 26d5903 (26d59037c6489e40dfa37bc704efc86c723a310b, parent: 70d86e3abeecf3a655264d9a716c5d08160176b7)

rust-timer · 2026-03-05T15:18:40Z

Finished benchmarking commit (26d5903): comparison URL.

Overall result: ❌ regressions - 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)	0.6%	[0.1%, 1.1%]	2
Regressions ❌ (secondary)	0.2%	[0.1%, 0.3%]	9
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	0.6%	[0.1%, 1.1%]	2

Max RSS (memory usage)

Results (primary -4.0%, secondary 3.8%)

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)	3.8%	[3.8%, 3.8%]	1
Improvements ✅ (primary)	-4.0%	[-5.3%, -3.1%]	3
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-4.0%	[-5.3%, -3.1%]	3

Cycles

Results (primary -1.3%)

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)	-1.3%	[-1.3%, -1.3%]	1
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-1.3%	[-1.3%, -1.3%]	1

Binary size

Results (primary 0.5%, 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)	1.2%	[0.0%, 2.3%]	3
Regressions ❌ (secondary)	0.3%	[0.0%, 0.4%]	12
Improvements ✅ (primary)	-0.0%	[-0.0%, -0.0%]	4
Improvements ✅ (secondary)	-0.0%	[-0.0%, -0.0%]	1
All ❌✅ (primary)	0.5%	[-0.0%, 2.3%]	7

Bootstrap: 480.787s -> 479.516s (-0.26%)
Artifact size: 395.02 MiB -> 395.00 MiB (-0.00%)

oli-obk · 2026-03-05T18:37:40Z

Fun. The entire previous improvement was from the PR this was based on

oli-obk · 2026-04-21T07:29:10Z

The large regression (cargo) is mostly in LLVM (along with lots of noisy changes). Maybe there's just more optimizations enabled by this change? idk

@rustbot ready

could also just close this PR if you think it's not worth it

cjgillot · 2026-05-14T22:53:11Z

r=me after rebase

…get type

…ansmute entirely because the types line up

rustbot · 2026-06-25T13:29:16Z

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.

rust-log-analyzer · 2026-06-25T15:02:55Z

The job x86_64-gnu-gcc-core-tests failed! Check out the build log: (web) (plain enhanced) (plain)

Click to see the possible cause of the failure (guessed by this bot)

   Compiling gimli v0.32.3
[RUSTC-TIMING] memchr test:false 11.115
[RUSTC-TIMING] gimli test:false 7.969
   Compiling addr2line v0.25.1
libgccjit.so: error: : gcc_jit_context_new_call: mismatching types for argument 3 of function "memcmp": assignment to param arg2 (type: size_t) from bitcast(loadedValue6, __int64_t  __attribute__((aligned(8)))) (type: __int64_t  __attribute__((aligned(8))))
libgccjit.so: error: : gcc_jit_context_new_cast: NULL rvalue
gcc_jit_rvalue_get_type: NULL rvalue
gcc_jit_type_get_pointer: NULL type
libgccjit.so: error: : gcc_jit_context_new_bitcast: NULL type
gcc_jit_rvalue_dereference: NULL rvalue
gcc_jit_lvalue_as_rvalue: NULL lvalue
gcc_jit_rvalue_get_type: NULL rvalue
gcc_jit_type_unqualified: NULL type
gcc_jit_type_get_aligned: NULL type
gcc_jit_type_get_pointer: NULL type
libgccjit.so: error: : gcc_jit_context_new_cast: NULL rvalue
gcc_jit_rvalue_dereference: NULL rvalue
libgccjit.so: error: : gcc_jit_block_add_assignment: NULL lvalue
libgccjit.so: error: : gcc_jit_context_new_call: mismatching types for argument 3 of function "memcmp": assignment to param arg2 (type: size_t) from bitcast(loadedValue6, __int64_t  __attribute__((aligned(8)))) (type: __int64_t  __attribute__((aligned(8))))
libgccjit.so: error: : gcc_jit_context_new_cast: NULL rvalue
gcc_jit_rvalue_get_type: NULL rvalue
gcc_jit_type_get_pointer: NULL type
libgccjit.so: error: : gcc_jit_context_new_bitcast: NULL type
gcc_jit_rvalue_dereference: NULL rvalue
gcc_jit_lvalue_as_rvalue: NULL lvalue
gcc_jit_rvalue_get_type: NULL rvalue
gcc_jit_type_unqualified: NULL type
gcc_jit_type_get_aligned: NULL type
gcc_jit_type_get_pointer: NULL type
libgccjit.so: error: : gcc_jit_context_new_cast: NULL rvalue
gcc_jit_rvalue_dereference: NULL rvalue
libgccjit.so: error: : gcc_jit_block_add_assignment: NULL lvalue
error: failed to build archive at `/checkout/obj/build/x86_64-unknown-linux-gnu/stage1-std/x86_64-unknown-linux-gnu/dist/build/addr2line/3b48cc3f07a65e65/out/libaddr2line-3b48cc3f07a65e65.rlib`: failed to open object file: No such file or directory (os error 2)

[RUSTC-TIMING] addr2line test:false 1.150
rustc exited with signal: 6 (SIGABRT) (core dumped)
error: could not compile `addr2line` (lib) due to 1 previous error

Caused by:
  process didn't exit successfully: `/checkout/obj/build/bootstrap/debug/rustc /checkout/obj/build/bootstrap/debug/rustc --crate-name addr2line --edition=2018 /cargo/registry/src/index.crates.io-1949cf8c6b5b557f/addr2line-0.25.1/src/lib.rs --error-format=json --json=diagnostic-rendered-ansi,artifacts,future-incompat --crate-type lib --emit=dep-info,metadata,link -Z embed-metadata=no -C opt-level=s -C embed-bitcode=no -C codegen-units=1 -Cembed-bitcode=yes -Zunstable-options -Cforce-frame-pointers=non-leaf -C overflow-checks=on --cfg 'feature="alloc"' --cfg 'feature="core"' --cfg 'feature="rustc-dep-of-std"' --check-cfg 'cfg(docsrs,test)' --check-cfg 'cfg(feature, values("all", "alloc", "bin", "cargo-all", "core", "cpp_demangle", "default", "fallible-iterator", "loader", "rustc-demangle", "rustc-dep-of-std", "smallvec", "std", "wasm"))' -C metadata=121783180a5b731d -C extra-filename=-3b48cc3f07a65e65 --out-dir /checkout/obj/build/x86_64-unknown-linux-gnu/stage1-std/x86_64-unknown-linux-gnu/dist/build/addr2line/3b48cc3f07a65e65/out --target x86_64-unknown-linux-gnu -C strip=debuginfo -L dependency=/checkout/obj/build/x86_64-unknown-linux-gnu/stage1-std/dist/build/cc/54a6600bab8cf9fc/out -L dependency=/checkout/obj/build/x86_64-unknown-linux-gnu/stage1-std/dist/build/compiler_builtins/b3edefae36c8c7c7/out -L dependency=/checkout/obj/build/x86_64-unknown-linux-gnu/stage1-std/dist/build/shlex/e43f9179e4b33b2b/out -L dependency=/checkout/obj/build/x86_64-unknown-linux-gnu/stage1-std/x86_64-unknown-linux-gnu/dist/build/addr2line/3b48cc3f07a65e65/out -L dependency=/checkout/obj/build/x86_64-unknown-linux-gnu/stage1-std/x86_64-unknown-linux-gnu/dist/build/alloc/49220ed811a06747/out -L dependency=/checkout/obj/build/x86_64-unknown-linux-gnu/stage1-std/x86_64-unknown-linux-gnu/dist/build/compiler_builtins/828632ac172df5b0/out -L dependency=/checkout/obj/build/x86_64-unknown-linux-gnu/stage1-std/x86_64-unknown-linux-gnu/dist/build/compiler_builtins/f815ffc9c670c233/out -L dependency=/checkout/obj/build/x86_64-unknown-linux-gnu/stage1-std/x86_64-unknown-linux-gnu/dist/build/core/2b9ece7490e9479d/out -L dependency=/checkout/obj/build/x86_64-unknown-linux-gnu/stage1-std/x86_64-unknown-linux-gnu/dist/build/gimli/929ad9b97a681af9/out -L dependency=/checkout/obj/build/x86_64-unknown-linux-gnu/stage1-std/x86_64-unknown-linux-gnu/dist/build/rustc-std-workspace-alloc/f57839810cfe131e/out -L dependency=/checkout/obj/build/x86_64-unknown-linux-gnu/stage1-std/x86_64-unknown-linux-gnu/dist/build/rustc-std-workspace-core/786a9c6b9c52478d/out -L dependency=/checkout/obj/build/x86_64-unknown-linux-gnu/stage1-std/dist/build/addr2line/3b48cc3f07a65e65/out --extern gimli=/checkout/obj/build/x86_64-unknown-linux-gnu/stage1-std/x86_64-unknown-linux-gnu/dist/build/gimli/929ad9b97a681af9/out/libgimli-929ad9b97a681af9.rmeta --extern alloc=/checkout/obj/build/x86_64-unknown-linux-gnu/stage1-std/x86_64-unknown-linux-gnu/dist/build/rustc-std-workspace-alloc/f57839810cfe131e/out/librustc_std_workspace_alloc-f57839810cfe131e.rmeta --extern core=/checkout/obj/build/x86_64-unknown-linux-gnu/stage1-std/x86_64-unknown-linux-gnu/dist/build/rustc-std-workspace-core/786a9c6b9c52478d/out/librustc_std_workspace_core-786a9c6b9c52478d.rmeta --cap-lints allow -Zannotate-moves '--check-cfg=cfg(feature,values(any()))' -Zunstable-options -Zmacro-backtrace -Csplit-debuginfo=off -Cprefer-dynamic -Zinline-mir -Zinline-mir-preserve-debug -Zmir_strip_debuginfo=locals-in-tiny-functions -Clink-args=-Wl,-z,origin '-Clink-args=-Wl,-rpath,$ORIGIN/../lib' -Alinker-messages '-Zcrate-attr=doc(html_root_url="https://doc.rust-lang.org/nightly/")' -Cpanic=abort -Zpanic-abort-tests -Z binary-dep-depinfo -L native=/checkout/obj/build/x86_64-unknown-linux-gnu/stage1-std/x86_64-unknown-linux-gnu/dist/build/compiler_builtins/f815ffc9c670c233/out` (exit status: 254)
warning: build failed, waiting for other jobs to finish...
[RUSTC-TIMING] compiler_builtins test:false 22.744
[RUSTC-TIMING] object test:false 8.671
[RUSTC-TIMING] core test:false 45.643
Bootstrap failed while executing `--stage 1 test library/coretests`

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For more information how to resolve CI failures of this job, visit this link.

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. T-libs Relevant to the library team, which will review and decide on the PR/issue. labels Feb 25, 2026

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Auto merge of #153085 - oli-obk:transmute_project, r=<try>

de634ba

GVN: transmute adts to their fields if a field projection is immediately transmuted anyway

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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 Feb 25, 2026

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Comment thread compiler/rustc_mir_transform/src/gvn.rs Outdated

Comment thread compiler/rustc_mir_transform/src/gvn.rs Outdated

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Auto merge of #153085 - oli-obk:transmute_project, r=<try>

26d5903

GVN: transmute adts to their fields if a field projection is immediately transmuted anyway

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Comment thread compiler/rustc_mir_transform/src/gvn.rs Outdated

oli-obk and others added 4 commits June 25, 2026 15:18

Turn field accesses into a transmute if followed by a transmute anyway

2e7c2f3

Add test for option transmutes

f0b3ce4

Also skip downcasts if the size of the enum matches the transmute tar…

ee0c910

…get type

Fix fast path and clarify similar variable names when skipping the tr…

d05dccd

…ansmute entirely because the types line up

oli-obk force-pushed the transmute_project branch from cf83de7 to d05dccd Compare June 25, 2026 13:29

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

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Overall result: ❌ regressions - please read the text below

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