003a3f8cd3
`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).
60 lines
1.5 KiB
Rust
60 lines
1.5 KiB
Rust
// compile-flags: -C no-prepopulate-passes -O
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#![crate_type = "lib"]
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pub enum E {
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A,
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B,
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C,
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}
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// CHECK-LABEL: @exhaustive_match
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#[no_mangle]
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pub fn exhaustive_match(e: E) -> u8 {
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// CHECK: switch{{.*}}, label %[[OTHERWISE:[a-zA-Z0-9_]+]] [
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// CHECK-NEXT: i[[TY:[0-9]+]] [[DISCR:[0-9]+]], label %[[A:[a-zA-Z0-9_]+]]
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// CHECK-NEXT: i[[TY:[0-9]+]] [[DISCR:[0-9]+]], label %[[B:[a-zA-Z0-9_]+]]
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// CHECK-NEXT: i[[TY:[0-9]+]] [[DISCR:[0-9]+]], label %[[C:[a-zA-Z0-9_]+]]
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// CHECK-NEXT: ]
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// CHECK: [[OTHERWISE]]:
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// CHECK-NEXT: unreachable
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//
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// CHECK: [[A]]:
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// CHECK-NEXT: store i8 0, {{i8\*|ptr}} %1, align 1
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// CHECK-NEXT: br label %[[EXIT:[a-zA-Z0-9_]+]]
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// CHECK: [[B]]:
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// CHECK-NEXT: store i8 1, {{i8\*|ptr}} %1, align 1
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// CHECK-NEXT: br label %[[EXIT]]
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// CHECK: [[C]]:
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// CHECK-NEXT: store i8 2, {{i8\*|ptr}} %1, align 1
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// CHECK-NEXT: br label %[[EXIT]]
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match e {
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E::A => 0,
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E::B => 1,
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E::C => 2,
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}
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}
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#[repr(u16)]
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pub enum E2 {
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A = 13,
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B = 42,
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}
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// For optimized code we produce a switch with an unreachable target as the `otherwise` so LLVM
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// knows the possible values. Compare with `src/test/codegen/match-unoptimized.rs`.
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// CHECK-LABEL: @exhaustive_match_2
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#[no_mangle]
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pub fn exhaustive_match_2(e: E2) -> u8 {
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// CHECK: switch i16 %{{.+}}, label %[[UNREACH:.+]] [
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// CHECK-NEXT: i16 13,
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// CHECK-NEXT: i16 42,
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// CHECK-NEXT: ]
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// CHECK: [[UNREACH]]:
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// CHECK-NEXT: unreachable
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match e {
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E2::A => 0,
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E2::B => 1,
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}
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}
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