Allow `transmute`s to produce `OperandValue`s instead of needing `alloca`s #109843

scottmcm · 2023-04-01T19:24:38Z

LLVM can usually optimize these away, but especially for things like transmutes of newtypes it's silly to generate the alloc+store+load at all when it's actually a nop at LLVM level.

rustbot · 2023-04-01T19:24:43Z

r? @WaffleLapkin

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scottmcm · 2023-04-01T21:27:30Z

Looks like I missed some ScalarPair cases
@rustbot author

scottmcm · 2023-04-01T22:59:32Z

Switched to https://doc.rust-lang.org/nightly/nightly-rustc/rustc_codegen_ssa/traits/trait.BuilderMethods.html#tymethod.load_operand which handles all the cases and is better than what I was doing before anyway
@rustbot ready

WaffleLapkin

I'm not exactly sure what's happening here, but here are some initial thoughts

compiler/rustc_codegen_ssa/src/mir/rvalue.rs

tests/codegen/intrinsics/transmute.rs

scottmcm · 2023-04-04T01:07:31Z

compiler/rustc_codegen_ssa/src/traits/type_.rs

+    /// For nearly all types this is the same as the [`backend_type`], however
+    /// `bool` (and other `0`-or-`1` values) are kept as [`BaseTypeMethods::type_i1`]
+    /// in registers but as [`BaseTypeMethods::type_i8`] in memory.


Your mention of uncertainty was a good prompt to write up some of what I learned as I was stumbling around trying to figure out how all this stuff worked 🙂

WaffleLapkin

Looks reasonable to me. r=me modulo the ZST question. (thanks for expanding docs!)

WaffleLapkin · 2023-04-04T14:13:18Z

compiler/rustc_codegen_ssa/src/mir/rvalue.rs

-        let size_in_bytes = src.layout.size.bytes();
-        if size_in_bytes == 0 {
-            // Nothing to write


Is there a particular reason to remove the ZST check?

Because, as I dug more, it was really only useful for my manual memcpy; OperandValue::store already does it:

rust/compiler/rustc_codegen_ssa/src/mir/operand.rs

Lines 285 to 289 in be8e5ba

// Avoid generating stores of zero-sized values, because the only way to have a zero-sized

// value is through `undef`, and store itself is useless.

if dest.layout.is_zst() {

return;

}

WaffleLapkin · 2023-04-04T14:23:09Z

tests/codegen/simd-intrinsic/simd-intrinsic-transmute-array.rs

+    // CHECK: %[[VAL:.+]] = load <4 x float>, {{ptr %x|.+>\* %.+}}, align 4
+    // CHECK: store <4 x float> %[[VAL:.+]], {{ptr %0|.+>\* %.+}}, align 16
+    unsafe { std::mem::transmute(x) }


Don't think it's a problem, but it's interesting how S(x) produces a memcpy while transmute produces load+store, while doing identical work (I think?).

Quite! When I saw this I brought it up with the portable-simd folks, since it seems that LLVM doesn't normalize it in IR in either direction, so they might want to experiment with which is better.

scottmcm · 2023-04-04T18:26:02Z

@bors r=WaffleLapkin

bors · 2023-04-04T18:26:04Z

📌 Commit 7afaa7ce1861481fdb0af9a6469b5b60e47787c5 has been approved by WaffleLapkin

It is now in the queue for this repository.

bors · 2023-04-04T22:12:09Z

⌛ Testing commit 7afaa7ce1861481fdb0af9a6469b5b60e47787c5 with merge 3913a2b4c2bbaf2dd5c5610ecfc6beea1ababd99...

bors · 2023-04-04T22:35:03Z

💔 Test failed - checks-actions

scottmcm · 2023-04-04T22:39:47Z

Of course, 32-bit catches me again 😭

@bors r-

bors · 2023-04-05T01:09:25Z

💔 Test failed - checks-actions

… `alloca`s LLVM can usually optimize these away, but especially for things like transmutes of newtypes it's silly to generate the `alloc`+`store`+`load` at all when it's actually a nop at LLVM level.

scottmcm · 2023-04-05T02:18:35Z

...and updated the test to not accidentally rely on standard library optimizations. Passed for -gnu-nopt in https://github.com/rust-lang/rust/actions/runs/4614082794/jobs/8156716838?pr=109843#step:26:1864

@bors r=WaffleLapkin rollup=iffy

bors · 2023-04-05T02:18:37Z

📌 Commit 9aa9a84 has been approved by WaffleLapkin

It is now in the queue for this repository.

bors · 2023-04-05T03:26:42Z

⌛ Testing commit 9aa9a84 with merge 8d321f7...

bors · 2023-04-05T05:47:26Z

☀️ Test successful - checks-actions
Approved by: WaffleLapkin
Pushing 8d321f7 to master...

rust-timer · 2023-04-05T07:53:29Z

Finished benchmarking commit (8d321f7): comparison URL.

Overall result: no relevant changes - no action needed

@rustbot label: -perf-regression

Instruction count

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

Max RSS (memory usage)

Results

This is a less reliable metric that may be of interest but was not used to determine the overall result at the top of this comment.

	mean	range	count
Regressions ❌ (primary)	-	-	0
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-0.2%	[-0.2%, -0.2%]	1
Improvements ✅ (secondary)	-2.3%	[-3.6%, -1.2%]	3
All ❌✅ (primary)	-0.2%	[-0.2%, -0.2%]	1

Cycles

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

scottmcm · 2023-04-05T08:36:40Z

Wow, more transmutes out there than I'd thought -- 73 relevant binary size improvements in debug, with no (even non-relevant) size regressions.

(Nothing relevant for opt size, though, unsurprisingly.)

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Allow transmutes to produce OperandValues instead of always using…

9aa9a84

… `alloca`s LLVM can usually optimize these away, but especially for things like transmutes of newtypes it's silly to generate the `alloc`+`store`+`load` at all when it's actually a nop at LLVM level.

scottmcm force-pushed the better-transmute branch from 462652d to 9aa9a84 Compare April 5, 2023 01:44

rustbot added A-testsuite Area: The testsuite used to check the correctness of rustc T-infra Relevant to the infrastructure team, which will review and decide on the PR/issue. labels Apr 5, 2023

scottmcm force-pushed the better-transmute branch from 53ac230 to 9aa9a84 Compare April 5, 2023 02:16

bors added S-waiting-on-bors Status: Waiting on bors to run and complete tests. Bors will change the label on completion. and removed S-waiting-on-author Status: This is awaiting some action (such as code changes or more information) from the author. labels Apr 5, 2023

bors added the merged-by-bors This PR was explicitly merged by bors. label Apr 5, 2023

bors merged commit 8d321f7 into rust-lang:master Apr 5, 2023
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scottmcm deleted the better-transmute branch April 5, 2023 05:48

scottmcm mentioned this pull request Apr 5, 2023

Missing optimization when transmuting enums #109958

Closed

matthiaskrgr mentioned this pull request Apr 6, 2023

LLVM_ERROR: invalid bitcast: transmute #110005

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ehuss mentioned this pull request Apr 14, 2023

Spurious CI failure: tests/run-make/jobserver-error #110321

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scottmcm mentioned this pull request Mar 12, 2024

Lower transmutes from int to pointer type as gep on null #121282

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Allow `transmute`s to produce `OperandValue`s instead of needing `alloca`s #109843

Allow `transmute`s to produce `OperandValue`s instead of needing `alloca`s #109843

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	// Avoid generating stores of zero-sized values, because the only way to have a zero-sized
	// value is through `undef`, and store itself is useless.
	if dest.layout.is_zst() {
	return;
	}

Allow transmutes to produce OperandValues instead of needing allocas #109843

Allow transmutes to produce OperandValues instead of needing allocas #109843

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Overall result: no relevant changes - no action needed

Instruction count

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scottmcm commented Apr 5, 2023

Allow `transmute`s to produce `OperandValue`s instead of needing `alloca`s #109843

Allow `transmute`s to produce `OperandValue`s instead of needing `alloca`s #109843

scottmcm Apr 4, 2023 •

edited