miri: improve and simplify overflow detection by RalfJung · Pull Request #69002 · rust-lang/rust

RalfJung · 2020-02-09T15:36:24Z

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

…y ops

RalfJung · 2020-02-09T15:38:18Z

+ Rem => {
 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));


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.

With control flow const prop we could stop propping in arms that are only reachable by a failing assert terminator.

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.

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.

wesleywiser · 2020-02-10T15:52:29Z

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

rust-timer · 2020-02-10T15:52:30Z

Awaiting bors try build completion

bors · 2020-02-10T15:52:40Z

⌛ Trying commit d6c5a04 with merge 5bf0888774eedd0302d15303b594839d4c70873e...

rust-timer · 2020-02-10T16:02:55Z

Awaiting bors try build completion

bors · 2020-02-10T16:03:08Z

⌛ Trying commit d6c5a04 with merge 0f7a6fbaf793dfb724765f4d67fe0fa0addb53fb...

bors · 2020-02-10T18:27:38Z

☀️ Try build successful - checks-azure
Build commit: 0f7a6fbaf793dfb724765f4d67fe0fa0addb53fb (0f7a6fbaf793dfb724765f4d67fe0fa0addb53fb)

wesleywiser · 2020-02-11T01:19:31Z

@rust-timer build 0f7a6fbaf793dfb724765f4d67fe0fa0addb53fb

rust-timer · 2020-02-11T01:19:33Z

Queued 0f7a6fbaf793dfb724765f4d67fe0fa0addb53fb with parent 4d1241f, future comparison URL.

RalfJung · 2020-02-11T07:40:03Z

Looks like the auto-queueing thing doesn't work any more?

wesleywiser · 2020-02-11T09:25:08Z

It's been buggy lately but I think it got confused when I edited my comment.

Perf looks unaffected.

@bors r=oli-obk,wesleywiser

bors · 2020-02-11T09:25:10Z

📌 Commit c561d23 has been approved by oli-obk,wesleywiser

RalfJung · 2020-02-11T09:32:03Z

It's been buggy lately

Is there a bug report for this? I can't find any at https://github.com/rust-lang-nursery/rustc-perf/issues

@oli-obk

…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

@ghost

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

@oli-obk

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

RalfJung added 4 commits February 9, 2020 15:41

miri: simplify singed operator overflow detection
202d401

bring back extra check for int_min%-1
28f85c6

miri: equip unary_op with overflow detection
7d2f6ae

const-prop: use overflowing_unary_op for overflowing checking of unar…
ae23f70
…y ops

rust-highfive assigned oli-obk Feb 9, 2020

rust-highfive added the S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. label Feb 9, 2020

RalfJung commented Feb 9, 2020
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Comment threadsrc/librustc_mir/interpret/operator.rs

RalfJung commented Feb 9, 2020
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Comment threadsrc/librustc_mir/interpret/operator.rs

RalfJung commented Feb 9, 2020
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Comment threadsrc/librustc_mir/transform/const_prop.rs

wesleywiser reviewed Feb 9, 2020
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Comment threadsrc/librustc_mir/transform/const_prop.rs

RalfJung added 3 commits February 10, 2020 09:56

add test that checks overflows on arithmetic operators
b434d7e

div/rem overflow tests: also test i128
1ddb050

some more tests for i128 oveflow behavior
d6c5a04

oli-obk reviewed Feb 10, 2020
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Comment threadsrc/librustc_mir/interpret/operator.rs Outdated

oli-obk approved these changes Feb 10, 2020
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remove outdated comment
c561d23

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-review Status: Awaiting review from the assignee but also interested parties. labels Feb 11, 2020

jonas-schievink mentioned this pull request Feb 11, 2020
Rollup of 11 pull requests #69056
Closed

jonas-schievink mentioned this pull request Feb 11, 2020
Rollup of 11 pull requests #69075
Closed

Dylan-DPC-zz mentioned this pull request Feb 12, 2020
Rollup of 7 pull requests #69105
Merged

bors merged commit c561d23 into rust-lang:masterFeb 13, 2020

RalfJung deleted the miri-op-overflow branch February 13, 2020 10:13

RalfJung mentioned this pull request Feb 13, 2020
miri: fix exact_div #69126
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wesleywiser commented Feb 10, 2020•
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