rust/src/test/codegen/match-optimized.rs, branch tryhttps://github.com/rust-lang/rust
http://git.dreamy.place/mirrors/rust/atom?h=try2023-01-11T09:32:08+00:00Move /src/test to /tests2023-01-11T09:32:08+00:00Albert Larsan74931857+albertlarsan68@users.noreply.github.com2023-01-05T08:13:28+00:00urn:sha1:cf2dff2b1e3fa55fa5415d524200070d0d7aacfeUse `br` instead of `switch` in more cases.2022-10-30T23:16:39+00:00Nicholas Nethercoten.nethercote@gmail.com2022-10-20T07:59:07+00:00urn:sha1:003a3f8cd3ea4594e2c2bc89c05562a202868ce0
`codegen_switchint_terminator` already uses `br` instead of `switch`
when there is one normal target plus the `otherwise` target. But there's
another common case with two normal targets and an `otherwise` target
that points to an empty unreachable BB. This comes up a lot when
switching on the tags of enums that use niches.
The pattern looks like this:
```
bb1: ; preds = %bb6
%3 = load i8, ptr %_2, align 1, !range !9, !noundef !4
%4 = sub i8 %3, 2
%5 = icmp eq i8 %4, 0
%_6 = select i1 %5, i64 0, i64 1
switch i64 %_6, label %bb3 [
i64 0, label %bb4
i64 1, label %bb2
]
bb3: ; preds = %bb1
unreachable
```
This commit adds code to convert the `switch` to a `br`:
```
bb1: ; preds = %bb6
%3 = load i8, ptr %_2, align 1, !range !9, !noundef !4
%4 = sub i8 %3, 2
%5 = icmp eq i8 %4, 0
%_6 = select i1 %5, i64 0, i64 1
%6 = icmp eq i64 %_6, 0
br i1 %6, label %bb4, label %bb2
bb3: ; No predecessors!
unreachable
```
This has a surprisingly large effect on compile times, with reductions
of 5% on debug builds of some crates. The reduction is all due to LLVM
taking less time. Maybe LLVM is just much better at handling `br` than
`switch`.
The resulting code is still suboptimal.
- The `icmp`, `select`, `icmp` sequence is silly, converting an `i1` to an `i64`
and back to an `i1`. But with the current code structure it's hard to avoid,
and LLVM will easily clean it up, in opt builds at least.
- `bb3` is usually now truly dead code (though not always, so it can't
be removed universally).