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`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). |
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