disable ConstProp of floats #113416

RalfJung · 2023-07-06T17:03:46Z

This works around issues such as #102403 and #113407. The issues still affect const evaluation, but at least they don't affect run-time code any more.

rustbot · 2023-07-06T17:03:52Z

r? @cjgillot

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rustbot · 2023-07-06T17:03:56Z

Some changes occurred to MIR optimizations

cc @rust-lang/wg-mir-opt

compiler/rustc_mir_transform/src/dataflow_const_prop.rs

cjgillot · 2023-07-06T17:26:51Z

Could you add tests for the issues?

RalfJung · 2023-07-06T17:28:51Z

compiler/rustc_mir_transform/src/dataflow_const_prop.rs

+            // This affects at least binary ops and casts, so just skip all rvalues.
+            // LLVM has a less buggy apfloat and will take care of const-propagation.
+            return ValueOrPlace::TOP;
+        }


Maybe it is better to short-circuit in handle_rvalue then. But it would really help if the docs said whether TOP or BOTTOM represents "could be any value, statically unknown"...

RalfJung · 2023-07-06T17:30:39Z

Could you add tests for the issues?

What kind of tests would be best suited here? ui tests asserting the values? Or mir-opt tests? The latter get blessed away so easily I don't dare rely on them for things like this...

RalfJung · 2023-07-07T10:59:34Z

tidy error: following path contains more than 1896 entries, you should move the test to some relevant subdirectory (current: 1898): /checkout/tests/ui/issues
some tidy checks failed

Please let me know where else I should put these tests. There is no "float" directory.

cjgillot · 2023-07-07T11:09:18Z

We have tests/ui/consts and tests/ui/mir, which both contain some regression tests for MIR opt bugs. A run-pass test in ui/consts may be better.

RalfJung · 2023-07-07T11:25:32Z

This has nothing to do with consts though. It's about const propagation (the optimization), not CTFE.

RalfJung · 2023-07-07T11:37:21Z

Ah, there is a const_prop folder. I'll put them there.

RalfJung · 2023-07-07T12:05:30Z

Looks like on CI we have an old LLVM where the bugs have not been fixed yet. Should I set -Copt-level to 0 so that only our own MIR opts get tested?

cjgillot · 2023-07-07T15:49:47Z

Ah, there is a const_prop folder. I'll put them there.

Good catch.

This has nothing to do with consts though. It's about const propagation (the optimization), not CTFE.

Could this manifest as a case where a const { some operation } would have a different result as doing the operation at runtime?

Should I set -Copt-level to 0 so that only our own MIR opts get tested?

-Cno-prepopulate-passes removes all LLVM action. -Copt-level=0 changes how some MIR opts behave.

RalfJung · 2023-07-07T16:05:00Z

Could this manifest as a case where a const { some operation } would have a different result as doing the operation at runtime?

Yes, the bugs remain unfixed in CTFE. We don't have an easy work-around for that case. I figured it'd be better to at least fix them for the cases where we can easily do that.

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---- [ui] tests/ui/const_prop/issue-113407.rs stdout ----

error: test run failed!
status: exit status: 101
command: cd "/checkout/obj/build/x86_64-unknown-linux-gnu/test/ui/const_prop/issue-113407" && RUST_TEST_THREADS="16" "/checkout/obj/build/x86_64-unknown-linux-gnu/test/ui/const_prop/issue-113407/a"
stdout: none
thread 'main' panicked at 'assertion failed: `(left == right)`
  left: `1`,
 right: `0`', fake-test-src-base/const_prop/issue-113407.rs:7:5
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

RalfJung · 2023-07-07T16:23:50Z

Looks like some LLVM passes are still running? The tests pass locally with newer LLVM...

RalfJung · 2023-07-26T20:48:16Z

Closing in favor of #113843, which properly fixes the issues. :)

rustbot assigned cjgillot Jul 6, 2023

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. labels Jul 6, 2023

RalfJung commented Jul 6, 2023

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

RalfJung mentioned this pull request Jul 6, 2023

Subnormal f64 to f32 cast is incorrect in CTFE #113407

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RalfJung force-pushed the no-float-const-prop branch from 063f53b to 18d6290 Compare July 6, 2023 17:15

disable ConstProp of floats

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RalfJung force-pushed the no-float-const-prop branch from 18d6290 to 44c6999 Compare July 6, 2023 17:28

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RalfJung force-pushed the no-float-const-prop branch 2 times, most recently from 0e4d70b to 02f1c3f Compare July 7, 2023 11:37

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RalfJung force-pushed the no-float-const-prop branch from 02f1c3f to 8a8ffe6 Compare July 7, 2023 11:46

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RalfJung added 2 commits July 7, 2023 18:06

add tests

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move misplaced tests

7153840

RalfJung force-pushed the no-float-const-prop branch from 8a8ffe6 to 7153840 Compare July 7, 2023 16:06

wesleywiser mentioned this pull request Jul 26, 2023

Replace in-tree rustc_apfloat with the new version of the crate #113843

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RalfJung deleted the no-float-const-prop branch March 9, 2024 17:52

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disable ConstProp of floats #113416

disable ConstProp of floats #113416

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