Auto merge of #45821 - djzin:unreachable-match-arms, r=nikomatsakis
always add an unreachable branch on matches to give more info to llvm As part of https://github.com/djzin/rustc-optimization I discovered that some simple enum optimizations (src/unary/three_valued_enum.rs and src/unary/four_valued_enum.rs in the repo) are not applied - and the reason for this is that we erase the info that the discriminant of an enum is one of the options by putting the last one in an "otherwise" branch. This patch adds an extra branch so that LLVM can know what the possibilities are for the discriminant, which fixes the three- and four- valued cases. Note that for whatever reason, this doesn't fix the case of 2 variants (most notably `Option` and `Result` have 2 variants) - a pass re-ordering might fix this or we may wish to add "assume" annotations on discriminants to force it to optimize.
This commit is contained in:
bors
commit
ff0f5de3b3
5 changed files with 103 additions and 31 deletions
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@ -205,7 +205,7 @@ impl<'a, 'gcx, 'tcx> Builder<'a, 'gcx, 'tcx> {
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if let Some(otherwise_block) = otherwise_block {
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targets.push(otherwise_block);
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} else {
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values.pop();
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targets.push(self.unreachable_block());
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}
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debug!("num_enum_variants: {}, tested variants: {:?}, variants: {:?}",
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num_enum_variants, values, variants);
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@ -307,6 +307,8 @@ struct Builder<'a, 'gcx: 'a+'tcx, 'tcx: 'a> {
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cached_resume_block: Option<BasicBlock>,
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/// cached block with the RETURN terminator
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cached_return_block: Option<BasicBlock>,
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/// cached block with the UNREACHABLE terminator
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cached_unreachable_block: Option<BasicBlock>,
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}
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struct CFG<'tcx> {
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@ -400,6 +402,11 @@ fn construct_fn<'a, 'gcx, 'tcx, A>(hir: Cx<'a, 'gcx, 'tcx>,
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TerminatorKind::Goto { target: return_block });
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builder.cfg.terminate(return_block, source_info,
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TerminatorKind::Return);
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// Attribute any unreachable codepaths to the function's closing brace
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if let Some(unreachable_block) = builder.cached_unreachable_block {
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builder.cfg.terminate(unreachable_block, source_info,
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TerminatorKind::Unreachable);
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}
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return_block.unit()
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}));
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assert_eq!(block, builder.return_block());
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@ -502,7 +509,8 @@ impl<'a, 'gcx, 'tcx> Builder<'a, 'gcx, 'tcx> {
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var_indices: NodeMap(),
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unit_temp: None,
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cached_resume_block: None,
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cached_return_block: None
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cached_return_block: None,
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cached_unreachable_block: None,
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};
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assert_eq!(builder.cfg.start_new_block(), START_BLOCK);
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@ -631,6 +639,17 @@ impl<'a, 'gcx, 'tcx> Builder<'a, 'gcx, 'tcx> {
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}
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}
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}
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fn unreachable_block(&mut self) -> BasicBlock {
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match self.cached_unreachable_block {
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Some(ub) => ub,
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None => {
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let ub = self.cfg.start_new_block();
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self.cached_unreachable_block = Some(ub);
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ub
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}
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}
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}
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}
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///////////////////////////////////////////////////////////////////////////
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44
src/test/codegen/match-optimizes-away.rs
Normal file
44
src/test/codegen/match-optimizes-away.rs
Normal file
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@ -0,0 +1,44 @@
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// Copyright 2017 The Rust Project Developers. See the COPYRIGHT
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// file at the top-level directory of this distribution and at
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// http://rust-lang.org/COPYRIGHT.
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//
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// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
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// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
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// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
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// option. This file may not be copied, modified, or distributed
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// except according to those terms.
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//
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// no-system-llvm
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// compile-flags: -O
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#![crate_type="lib"]
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pub enum Three { First, Second, Third }
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use Three::*;
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pub enum Four { First, Second, Third, Fourth }
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use Four::*;
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#[no_mangle]
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pub fn three_valued(x: Three) -> Three {
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// CHECK-LABEL: @three_valued
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// CHECK-NEXT: {{^.*:$}}
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// CHECK-NEXT: ret i8 %0
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match x {
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First => First,
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Second => Second,
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Third => Third,
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}
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}
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#[no_mangle]
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pub fn four_valued(x: Four) -> Four {
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// CHECK-LABEL: @four_valued
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// CHECK-NEXT: {{^.*:$}}
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// CHECK-NEXT: ret i8 %0
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match x {
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First => First,
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Second => Second,
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Third => Third,
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Fourth => Fourth,
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}
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}
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@ -21,12 +21,15 @@ pub enum E {
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#[no_mangle]
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pub fn exhaustive_match(e: E) {
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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 %[[TRUE:[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: ]
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// CHECK: [[TRUE]]:
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// CHECK: [[A]]:
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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: br label %[[EXIT:[a-zA-Z0-9_]+]]
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// CHECK: [[OTHERWISE]]:
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// CHECK-NEXT: br label %[[EXIT:[a-zA-Z0-9_]+]]
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// CHECK-NEXT: unreachable
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match e {
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E::A => (),
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E::B => (),
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@ -54,24 +54,24 @@ fn main() {
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// ...
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// _2 = std::option::Option<i32>::Some(const 42i32,);
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// _5 = discriminant(_2);
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// switchInt(_5) -> [0isize: bb5, otherwise: bb3];
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// switchInt(_5) -> [0isize: bb5, 1isize: bb3, otherwise: bb7];
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// }
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// bb1: { // arm1
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// StorageLive(_7);
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// _7 = _3;
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// _1 = (const 1i32, _7);
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// StorageDead(_7);
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// goto -> bb11;
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// goto -> bb12;
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// }
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// bb2: { // binding3(empty) and arm3
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// _1 = (const 3i32, const 3i32);
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// goto -> bb11;
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// goto -> bb12;
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// }
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// bb3: {
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// falseEdges -> [real: bb7, imaginary: bb4]; //pre_binding1
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// falseEdges -> [real: bb8, imaginary: bb4]; //pre_binding1
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// }
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// bb4: {
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// falseEdges -> [real: bb10, imaginary: bb5]; //pre_binding2
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// falseEdges -> [real: bb11, imaginary: bb5]; //pre_binding2
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// }
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// bb5: {
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// falseEdges -> [real: bb2, imaginary: bb6]; //pre_binding3
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@ -79,28 +79,31 @@ fn main() {
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// bb6: {
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// unreachable;
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// }
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// bb7: { // binding1 and guard
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// bb7: {
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// unreachable;
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// }
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// bb8: { // binding1 and guard
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// StorageLive(_3);
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// _3 = ((_2 as Some).0: i32);
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// StorageLive(_6);
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// _6 = const guard() -> bb8;
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// _6 = const guard() -> bb9;
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// }
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// bb8: { // end of guard
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// switchInt(_6) -> [0u8: bb9, otherwise: bb1];
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// bb9: { // end of guard
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// switchInt(_6) -> [0u8: bb10, otherwise: bb1];
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// }
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// bb9: { // to pre_binding2
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// bb10: { // to pre_binding2
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// falseEdges -> [real: bb4, imaginary: bb4];
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// }
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// bb10: { // bindingNoLandingPads.before.mir2 and arm2
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// bb11: { // bindingNoLandingPads.before.mir2 and arm2
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// StorageLive(_4);
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// _4 = ((_2 as Some).0: i32);
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// StorageLive(_8);
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// _8 = _4;
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// _1 = (const 2i32, _8);
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// StorageDead(_8);
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// goto -> bb11;
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// goto -> bb12;
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// }
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// bb11: {
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// bb12: {
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// ...
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// return;
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// }
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@ -111,53 +114,56 @@ fn main() {
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// ...
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// _2 = std::option::Option<i32>::Some(const 42i32,);
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// _5 = discriminant(_2);
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// switchInt(_5) -> [0isize: bb4, otherwise: bb3];
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// switchInt(_5) -> [0isize: bb4, 1isize: bb3, otherwise: bb7];
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// }
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// bb1: { // arm1
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// StorageLive(_7);
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// _7 = _3;
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// _1 = (const 1i32, _7);
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// StorageDead(_7);
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// goto -> bb11;
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// goto -> bb12;
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// }
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// bb2: { // binding3(empty) and arm3
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// _1 = (const 3i32, const 3i32);
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// goto -> bb11;
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// goto -> bb12;
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// }
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// bb3: {
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// falseEdges -> [real: bb7, imaginary: bb4]; //pre_binding1
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// falseEdges -> [real: bb8, imaginary: bb4]; //pre_binding1
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// }
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// bb4: {
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// falseEdges -> [real: bb2, imaginary: bb5]; //pre_binding2
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// }
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// bb5: {
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// falseEdges -> [real: bb10, imaginary: bb6]; //pre_binding3
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// falseEdges -> [real: bb11, imaginary: bb6]; //pre_binding3
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// }
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// bb6: {
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// unreachable;
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// }
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// bb7: { // binding1 and guard
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// bb7: {
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// unreachable;
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// }
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// bb8: { // binding1 and guard
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// StorageLive(_3);
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// _3 = ((_2 as Some).0: i32);
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// StorageLive(_6);
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// _6 = const guard() -> bb8;
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// _6 = const guard() -> bb9;
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// }
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// bb8: { // end of guard
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// switchInt(_6) -> [0u8: bb9, otherwise: bb1];
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// bb9: { // end of guard
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// switchInt(_6) -> [0u8: bb10, otherwise: bb1];
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// }
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// bb9: { // to pre_binding2
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// bb10: { // to pre_binding2
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// falseEdges -> [real: bb5, imaginary: bb4];
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// }
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// bb10: { // binding2 and arm2
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// bb11: { // binding2 and arm2
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// StorageLive(_4);
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// _4 = ((_2 as Some).0: i32);
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// StorageLive(_8);
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// _8 = _4;
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// _1 = (const 2i32, _8);
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// StorageDead(_8);
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// goto -> bb11;
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// goto -> bb12;
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// }
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// bb11: {
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// bb12: {
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// ...
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// return;
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// }
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