miri: improve and simplify overflow detection #69002
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| if r == -1 && l == (1 << (size.bits() - 1)) { | ||
| return Ok((Scalar::from_uint(l, size), true, left_layout.ty)); | ||
| return Ok((Scalar::from_int(0, size), true, left_layout.ty)); |
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I think what this used to do for Rem is plain wrong -- it returned the wrong result. However, even in release builds, remainder is checked for overflow. So, during normal execution in CTFE/Miri, the overflowing case here is unreachable. It is reachable in const-prop, which however does not care about the return value, just about whether there is an overflow.
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With control flow const prop we could stop propping in arms that are only reachable by a failing assert terminator.
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We could also declare int_min / -1 and int_min % -1 to be UB. This is consistent with MIR building always checking that, and it should resolve the rem/div part of #69020.
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Though OTOH, it is probably more consistent to handle all overflow the same way... so maybe not. I hope to play with this next weekend.
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Let's do a perf run just to be sure there's no regressions and then we can land this. @bors try @rust-timer queue |
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Awaiting bors try build completion |
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⌛ Trying commit d6c5a04 with merge 5bf0888774eedd0302d15303b594839d4c70873e... |
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Awaiting bors try build completion |
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⌛ Trying commit d6c5a04 with merge 0f7a6fbaf793dfb724765f4d67fe0fa0addb53fb... |
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☀️ Try build successful - checks-azure |
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@rust-timer build 0f7a6fbaf793dfb724765f4d67fe0fa0addb53fb |
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Queued 0f7a6fbaf793dfb724765f4d67fe0fa0addb53fb with parent 4d1241f, future comparison URL. |
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Looks like the auto-queueing thing doesn't work any more? |
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It's been buggy lately but I think it got confused when I edited my comment. Perf looks unaffected. @bors r=oli-obk,wesleywiser |
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📌 Commit c561d23 has been approved by |
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Is there a bug report for this? I can't find any at https://github.com/rust-lang-nursery/rustc-perf/issues |
…k,wesleywiser miri: improve and simplify overflow detection This simplifies the overflow detection for signed binary operators, and adds overflow detection to unary operators so that const-prop doesn't have to crudely hand-roll that. It also fixes some bugs in the operator implementation that however, I think, were not observable. r? @oli-obk @wesleywiser
Rollup of 7 pull requests Successful merges: - #67954 (Support new LLVM pass manager) - #68981 ( Account for type params on method without parentheses) - #69002 (miri: improve and simplify overflow detection) - #69038 (Add initial debug fmt for Backtrace) - #69040 (Cleanup SGX entry code) - #69086 (Update compiler-builtins to 0.1.25) - #69095 (Minified theme check) Failed merges: r? @ghost
miri: fix exact_div Turns out `exact_div` was relying on the broken behavior of `Rem` for `int_min % -1` that was fixed in rust-lang#69002. This PR fixes `exact_div`. Inside rustc, `exact_div` is only used in a single place where the divisor is always positive (in `ptr_offset_from`), so we cannot test the fix in rustc. The Miri test suite covers this through the `exact_div` intrinsic, though (and it is how I found out). One step to rust-lang#69117 (then we also need to address build failures introduced by rust-lang#68969) r? @oli-obk
This simplifies the overflow detection for signed binary operators, and adds overflow detection to unary operators so that const-prop doesn't have to crudely hand-roll that.
It also fixes some bugs in the operator implementation that however, I think, were not observable.
r? @oli-obk @wesleywiser