[EXPERIMENT] Don't monomorphize things that are unused due to if <T as Trait>::CONST

[scottmcm]

Fixes #85836

MIR optimizations have long meant that we do this for concrete constants, but with this change it also applies for things like <T as TrustedRandomAccess>::MAY_HAVE_SIDE_EFFECT.

There's likely not enough of this stuff for the extra work to be worth it, but I'm curious to see anyway.

r? @ghost

[scottmcm]

Queue's empty, so...

@bors try @rust-timer queue

[rust-timer]

Awaiting bors try build completion.

@rustbot label: +S-waiting-on-perf

[bors]

⌛ Trying commit 568fe20 with merge 258c6a0d164d6864d65295e968956d4cc6e246f2...

[bors]

💔 Test failed - checks-actions

[the8472]

MAY_HAVE_SIDE_EFFECT doesn't seem like a particularly juicy target since it's mostly used in Zip.

The following could be a juicier one if it were extracted to an associated const if that works together with specialization

rust/library/alloc/src/vec/source_iter_marker.rs

Lines 24 to 28 in 862962b

	if mem::size_of::<T>() == 0
	|| mem::size_of::<T>()
	!= mem::size_of::<<<I as SourceIter>::Source as AsIntoIter>::Item>()
	|| mem::align_of::<T>()
	!= mem::align_of::<<<I as SourceIter>::Source as AsIntoIter>::Item>()

I can prepare a PR for that if it helps.

[scottmcm]

Hmm, interesting that the try failed. I built a full stage2 locally without issue. Oh well, TBH I was surprised that just referencing these things "worked", so I'll take a look at removing the references to them too. Emitting less LLVM is always good.

--

@the8472 If you're excited to I'd happily take it in the branch, but this is still experimental enough that I can't honestly say that you should. It's still perfectly likely that this will go nowhere, so I wouldn't want you to waste time.

Honestly this PR might not be worth it until inline consts work so that could just be

        if const {
            mem::size_of::<T>() == 0
                || mem::size_of::<T>()
                    != mem::size_of::<<<I as SourceIter>::Source as AsIntoIter>::Item>()
                || mem::align_of::<T>()
                    != mem::align_of::<<<I as SourceIter>::Source as AsIntoIter>::Item>()
        } {
            // fallback to more generic implementations
            return SpecFromIterNested::from_iter(iterator);
        }

instead, and not need a whole bunch of these little one-use traits.

But right now they're using the basic "sugar for const item" implementation strategy which doesn't allow generic parameters to be referenced, making them unhelpful here.

(The context for this PR was seeing if I could get #85836 working.)

[the8472]

associated const PR over at scottmcm#3

[scottmcm]

@bors try @rust-timer queue

[rust-timer]

Awaiting bors try build completion.

@rustbot label: +S-waiting-on-perf

[bors]

⌛ Trying commit b728f16603191d29dda030521911a25c2e328c8d with merge 813a1d70587279815e909f5fc757d2e02322392e...

[bors]

☀️ Try build successful - checks-actions
Build commit: 813a1d70587279815e909f5fc757d2e02322392e (813a1d70587279815e909f5fc757d2e02322392e)

[rust-timer]

Queued 813a1d70587279815e909f5fc757d2e02322392e with parent 9adfd9d, future comparison URL.

[rust-timer]

Finished benchmarking commit (813a1d70587279815e909f5fc757d2e02322392e): comparison url.

Summary: This change led to large relevant mixed results 🤷 in compiler performance.

• Large improvement in instruction counts (up to -1.9% on full builds of inflate)
• Large regression in instruction counts (up to 3.8% on incr-unchanged builds of deep-vector)

If you disagree with this performance assessment, please file an issue in rust-lang/rustc-perf.

Benchmarking this pull request likely means that it is perf-sensitive, so we're automatically marking it as not fit for rolling up. While you can manually mark this PR as fit for rollup, we strongly recommend not doing so since this PR led to changes in compiler perf.

Next Steps: If you can justify the regressions found in this try perf run, please indicate this with @rustbot label: +perf-regression-triaged along with sufficient written justification. If you cannot justify the regressions please fix the regressions and do another perf run. If the next run shows neutral or positive results, the label will be automatically removed.

@bors rollup=never
@rustbot label: +S-waiting-on-review -S-waiting-on-perf +perf-regression

[rust-log-analyzer]

The job x86_64-gnu-llvm-12 failed! Check out the build log: (web) (plain)

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

.................F...............i.................................
failures:
Some tests failed in compiletest suite=mir-opt mode=mir-opt host=x86_64-unknown-linux-gnu target=i686-unknown-linux-gnu

---- [mir-opt] mir-opt/matches_reduce_branches.rs stdout ----
1 - // MIR for `match_nested_if` before MatchBranchSimplification
2 + // MIR for `match_nested_if` after MatchBranchSimplification
+   
4   fn match_nested_if() -> bool {
4   fn match_nested_if() -> bool {
5       let mut _0: bool;                    // return place in scope 0 at $DIR/matches_reduce_branches.rs:39:25: 39:29
6       let _1: bool;                        // in scope 0 at $DIR/matches_reduce_branches.rs:40:9: 40:12
13       scope 1 {
13       scope 1 {
14           debug val => _1;                 // in scope 1 at $DIR/matches_reduce_branches.rs:40:9: 40:12
- 
+   
17       bb0: {
17       bb0: {
18           StorageLive(_1);                 // scope 0 at $DIR/matches_reduce_branches.rs:40:9: 40:12
19           StorageLive(_2);                 // scope 0 at $DIR/matches_reduce_branches.rs:41:18: 41:81

24                                            // + span: $DIR/matches_reduce_branches.rs:41:24: 41:31
25                                            // + literal: Const { ty: fn() -> bool {unknown}, val: Value(Scalar(<ZST>)) }
- 
+   
28       bb1: {
28       bb1: {
29 -         switchInt(move _4) -> [false: bb3, otherwise: bb2]; // scope 0 at $DIR/matches_reduce_branches.rs:41:24: 41:33
30 -     }
- -
+ - 
32 -     bb2: {
32 -     bb2: {
33 -         _3 = const true;                 // scope 0 at $DIR/matches_reduce_branches.rs:41:36: 41:40
34 -         goto -> bb4;                     // scope 0 at $DIR/matches_reduce_branches.rs:41:21: 41:57
35 -     }
- -
+ - 
37 -     bb3: {
37 -     bb3: {
38 -         _3 = const false;                // scope 0 at $DIR/matches_reduce_branches.rs:41:50: 41:55
39 -         goto -> bb4;                     // scope 0 at $DIR/matches_reduce_branches.rs:41:21: 41:57
40 -     }
- -
+ - 
42 -     bb4: {
42 -     bb4: {
43 +         StorageLive(_5);                 // scope 0 at $DIR/matches_reduce_branches.rs:41:24: 41:33
44 +         _5 = move _4;                    // scope 0 at $DIR/matches_reduce_branches.rs:41:24: 41:33

47           StorageDead(_4);                 // scope 0 at $DIR/matches_reduce_branches.rs:41:56: 41:57
48 -         switchInt(move _3) -> [false: bb6, otherwise: bb5]; // scope 0 at $DIR/matches_reduce_branches.rs:41:21: 41:57
49 -     }
+ - 
51 -     bb5: {
51 -     bb5: {
52 -         _2 = const true;                 // scope 0 at $DIR/matches_reduce_branches.rs:41:60: 41:64
53 -         goto -> bb7;                     // scope 0 at $DIR/matches_reduce_branches.rs:41:18: 41:81
54 -     }
- -
+ - 
56 -     bb6: {
56 -     bb6: {
57 -         _2 = const false;                // scope 0 at $DIR/matches_reduce_branches.rs:41:74: 41:79
58 -         goto -> bb7;                     // scope 0 at $DIR/matches_reduce_branches.rs:41:18: 41:81
59 -     }
- -
+ - 
61 -     bb7: {
61 -     bb7: {
62 +         StorageLive(_6);                 // scope 0 at $DIR/matches_reduce_branches.rs:41:21: 41:57
63 +         _6 = move _3;                    // scope 0 at $DIR/matches_reduce_branches.rs:41:21: 41:57

66           StorageDead(_3);                 // scope 0 at $DIR/matches_reduce_branches.rs:41:80: 41:81
67 -         switchInt(move _2) -> [false: bb9, otherwise: bb8]; // scope 0 at $DIR/matches_reduce_branches.rs:41:18: 41:81
68 -     }
+ - 
70 -     bb8: {
70 -     bb8: {
71 +         StorageLive(_7);                 // scope 0 at $DIR/matches_reduce_branches.rs:41:18: 41:81
72 +         _7 = move _2;                    // scope 0 at $DIR/matches_reduce_branches.rs:41:18: 41:81

74 -         _1 = const true;                 // scope 0 at $DIR/matches_reduce_branches.rs:47:13: 47:17
75 -         goto -> bb10;                    // scope 0 at $DIR/matches_reduce_branches.rs:47:13: 47:17
76 -     }
+ - 
78 -     bb9: {
78 -     bb9: {
79 -         StorageDead(_2);                 // scope 0 at $DIR/matches_reduce_branches.rs:45:9: 45:10
80 -         _1 = const false;                // scope 0 at $DIR/matches_reduce_branches.rs:49:14: 49:19

81 -         goto -> bb10;                    // scope 0 at $DIR/matches_reduce_branches.rs:49:14: 49:19
82 -     }
+ - 
84 -     bb10: {
84 -     bb10: {
85 +         _1 = Ne(_7, const false);        // scope 0 at $DIR/matches_reduce_branches.rs:49:14: 49:19
86 +         StorageDead(_7);                 // scope 0 at $DIR/matches_reduce_branches.rs:41:18: 41:81

thread '[mir-opt] mir-opt/matches_reduce_branches.rs' panicked at 'Actual MIR output differs from expected MIR output /checkout/src/test/mir-opt/matches_reduce_branches.match_nested_if.MatchBranchSimplification.32bit.diff', src/tools/compiletest/src/runtest.rs:3357:25


failures:
    [mir-opt] mir-opt/matches_reduce_branches.rs
    [mir-opt] mir-opt/matches_reduce_branches.rs

test result: FAILED. 162 passed; 1 failed; 4 ignored; 0 measured; 0 filtered out; finished in 2.26s



command did not execute successfully: "/checkout/obj/build/x86_64-unknown-linux-gnu/stage0-tools-bin/compiletest" "--compile-lib-path" "/checkout/obj/build/x86_64-unknown-linux-gnu/stage2/lib" "--run-lib-path" "/checkout/obj/build/x86_64-unknown-linux-gnu/stage2/lib/rustlib/i686-unknown-linux-gnu/lib" "--rustc-path" "/checkout/obj/build/x86_64-unknown-linux-gnu/stage2/bin/rustc" "--src-base" "/checkout/src/test/mir-opt" "--build-base" "/checkout/obj/build/x86_64-unknown-linux-gnu/test/mir-opt" "--stage-id" "stage2-i686-unknown-linux-gnu" "--suite" "mir-opt" "--mode" "mir-opt" "--target" "i686-unknown-linux-gnu" "--host" "x86_64-unknown-linux-gnu" "--llvm-filecheck" "/usr/lib/llvm-12/bin/FileCheck" "--nodejs" "/usr/bin/node" "--linker" "cc" "--host-rustcflags" "-Crpath -O -Cdebuginfo=0  -Lnative=/checkout/obj/build/x86_64-unknown-linux-gnu/native/rust-test-helpers" "--target-rustcflags" "-Crpath -O -Cdebuginfo=0  -Lnative=/checkout/obj/build/i686-unknown-linux-gnu/native/rust-test-helpers" "--docck-python" "/usr/bin/python3" "--lldb-python" "/usr/bin/python3" "--gdb" "/usr/bin/gdb" "--quiet" "--llvm-version" "12.0.0" "--llvm-components" "aarch64 aarch64asmparser aarch64codegen aarch64desc aarch64disassembler aarch64info aarch64utils aggressiveinstcombine all all-targets amdgpu amdgpuasmparser amdgpucodegen amdgpudesc amdgpudisassembler amdgpuinfo amdgpuutils analysis arm armasmparser armcodegen armdesc armdisassembler arminfo armutils asmparser asmprinter avr avrasmparser avrcodegen avrdesc avrdisassembler avrinfo binaryformat bitreader bitstreamreader bitwriter bpf bpfasmparser bpfcodegen bpfdesc bpfdisassembler bpfinfo cfguard codegen core coroutines coverage debuginfocodeview debuginfodwarf debuginfogsym debuginfomsf debuginfopdb demangle dlltooldriver dwarflinker engine executionengine extensions filecheck frontendopenacc frontendopenmp fuzzmutate globalisel hellonew hexagon hexagonasmparser hexagoncodegen hexagondesc hexagondisassembler hexagoninfo instcombine instrumentation interfacestub interpreter ipo irreader jitlink lanai lanaiasmparser lanaicodegen lanaidesc lanaidisassembler lanaiinfo libdriver lineeditor linker lto mc mca mcdisassembler mcjit mcparser mips mipsasmparser mipscodegen mipsdesc mipsdisassembler mipsinfo mirparser msp430 msp430asmparser msp430codegen msp430desc msp430disassembler msp430info native nativecodegen nvptx nvptxcodegen nvptxdesc nvptxinfo objcarcopts object objectyaml option orcjit orcshared orctargetprocess passes perfjitevents powerpc powerpcasmparser powerpccodegen powerpcdesc powerpcdisassembler powerpcinfo profiledata remarks riscv riscvasmparser riscvcodegen riscvdesc riscvdisassembler riscvinfo runtimedyld scalaropts selectiondag sparc sparcasmparser sparccodegen sparcdesc sparcdisassembler sparcinfo support symbolize systemz systemzasmparser systemzcodegen systemzdesc systemzdisassembler systemzinfo tablegen target textapi transformutils vectorize webassembly webassemblyasmparser webassemblycodegen webassemblydesc webassemblydisassembler webassemblyinfo windowsmanifest x86 x86asmparser x86codegen x86desc x86disassembler x86info xcore xcorecodegen xcoredesc xcoredisassembler xcoreinfo xray" "--cc" "" "--cxx" "" "--cflags" "" "--adb-path" "adb" "--adb-test-dir" "/data/tmp/work" "--android-cross-path" "" "--channel" "nightly" "--color" "always"


Build completed unsuccessfully in 0:01:45