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Auto merge of #103331 - nnethercote:convert-switch-to-br, r=scottmcm

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Use `br` instead of `switch` in more cases.

`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).

r? `@scottmcm`
This commit is contained in:
bors 2022-10-31 00:40:32 +00:00
commit d726c8467c
5 changed files with 116 additions and 31 deletions
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30
compiler/rustc_codegen_ssa/src/mir/block.rs
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@ -17,6 +17,7 @@ use rustc_middle::mir::{self, AssertKind, SwitchTargets};
use rustc_middle::ty::layout::{HasTyCtxt, LayoutOf};
use rustc_middle::ty::print::{with_no_trimmed_paths, with_no_visible_paths};
use rustc_middle::ty::{self, Instance, Ty, TypeVisitable};
use rustc_session::config::OptLevel;
use rustc_span::source_map::Span;
use rustc_span::{sym, Symbol};
use rustc_symbol_mangling::typeid::typeid_for_fnabi;
@ -286,12 +287,13 @@ impl<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>> FunctionCx<'a, 'tcx, Bx> {
assert_eq!(discr.layout.ty, switch_ty);
let mut target_iter = targets.iter();
if target_iter.len() == 1 {
// If there are two targets (one conditional, one fallback), emit br instead of switch
// If there are two targets (one conditional, one fallback), emit `br` instead of
// `switch`.
let (test_value, target) = target_iter.next().unwrap();
let lltrue = helper.llbb_with_cleanup(self, target);
let llfalse = helper.llbb_with_cleanup(self, targets.otherwise());
if switch_ty == bx.tcx().types.bool {
// Don't generate trivial icmps when switching on bool
// Don't generate trivial icmps when switching on bool.
match test_value {
0 => bx.cond_br(discr.immediate(), llfalse, lltrue),
1 => bx.cond_br(discr.immediate(), lltrue, llfalse),
@ -303,6 +305,30 @@ impl<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>> FunctionCx<'a, 'tcx, Bx> {
let cmp = bx.icmp(IntPredicate::IntEQ, discr.immediate(), llval);
bx.cond_br(cmp, lltrue, llfalse);
}
} else if self.cx.sess().opts.optimize == OptLevel::No
&& target_iter.len() == 2
&& self.mir[targets.otherwise()].is_empty_unreachable()
{
// In unoptimized builds, if there are two normal targets and the `otherwise` target is
// an unreachable BB, emit `br` instead of `switch`. This leaves behind the unreachable
// BB, which will usually (but not always) be dead code.
//
// Why only in unoptimized builds?
// - In unoptimized builds LLVM uses FastISel which does not support switches, so it
// must fall back to the to the slower SelectionDAG isel. Therefore, using `br` gives
// significant compile time speedups for unoptimized builds.
// - In optimized builds the above doesn't hold, and using `br` sometimes results in
// worse generated code because LLVM can no longer tell that the value being switched
// on can only have two values, e.g. 0 and 1.
//
let (test_value1, target1) = target_iter.next().unwrap();
let (_test_value2, target2) = target_iter.next().unwrap();
let ll1 = helper.llbb_with_cleanup(self, target1);
let ll2 = helper.llbb_with_cleanup(self, target2);
let switch_llty = bx.immediate_backend_type(bx.layout_of(switch_ty));
let llval = bx.const_uint_big(switch_llty, test_value1);
let cmp = bx.icmp(IntPredicate::IntEQ, discr.immediate(), llval);
bx.cond_br(cmp, ll1, ll2);
} else {
bx.switch(
discr.immediate(),

5
compiler/rustc_middle/src/mir/mod.rs
View File

@ -1186,6 +1186,11 @@ impl<'tcx> BasicBlockData<'tcx> {
pub fn visitable(&self, index: usize) -> &dyn MirVisitable<'tcx> {
if index < self.statements.len() { &self.statements[index] } else { &self.terminator }
}
/// Does the block have no statements and an unreachable terminator?
pub fn is_empty_unreachable(&self) -> bool {
self.statements.is_empty() && matches!(self.terminator().kind, TerminatorKind::Unreachable)
}
}
impl<O> AssertKind<O> {

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

23
src/test/codegen/match-unoptimized.rs Normal file
View File

@ -0,0 +1,23 @@
// compile-flags: -C no-prepopulate-passes -Copt-level=0
#![crate_type = "lib"]
#[repr(u16)]
pub enum E2 {
A = 13,
B = 42,
}
// For unoptimized code we produce a `br` instead of a `switch`. Compare with
// `src/test/codegen/match-optimized.rs`
// CHECK-LABEL: @exhaustive_match_2
#[no_mangle]
pub fn exhaustive_match_2(e: E2) -> u8 {
// CHECK: %[[CMP:.+]] = icmp eq i16 %{{.+}}, 13
// CHECK-NEXT: br i1 %[[CMP:.+]],
match e {
E2::A => 0,
E2::B => 1,
}
}

29
src/test/codegen/match.rs
View File

@ -1,29 +0,0 @@
// compile-flags: -C no-prepopulate-passes
#![crate_type = "lib"]
pub enum E {
A,
B,
}
// CHECK-LABEL: @exhaustive_match
#[no_mangle]
pub fn exhaustive_match(e: E) -> u8 {
// CHECK: switch{{.*}}, label %[[OTHERWISE:[a-zA-Z0-9_]+]] [
// CHECK-NEXT: i[[TY:[0-9]+]] [[DISCR:[0-9]+]], label %[[A:[a-zA-Z0-9_]+]]
// CHECK-NEXT: i[[TY:[0-9]+]] [[DISCR:[0-9]+]], label %[[B:[a-zA-Z0-9_]+]]
// CHECK-NEXT: ]
// CHECK: [[OTHERWISE]]:
// CHECK-NEXT: unreachable
// CHECK: [[A]]:
// CHECK-NEXT: store i8 0, {{i8\*|ptr}} %1, align 1
// CHECK-NEXT: br label %[[EXIT:[a-zA-Z0-9_]+]]
// CHECK: [[B]]:
// CHECK-NEXT: store i8 1, {{i8\*|ptr}} %1, align 1
// CHECK-NEXT: br label %[[EXIT:[a-zA-Z0-9_]+]]
match e {
E::A => 0,
E::B => 1,
}
}