X-Git-Url: https://git.lizzy.rs/?a=blobdiff_plain;ds=sidebyside;f=compiler%2Frustc_codegen_ssa%2Fsrc%2Fmir%2Fblock.rs;h=0802067cde65d85478612cfa1021312d89cfe3f3;hb=75c239402c8fafc89246a26bd066d6ff647e3794;hp=29b7c9b0a8832af029fd84c07812acc057143d9f;hpb=b03fa1a3fe02cd0087982cc22bda7598a9fc39eb;p=rust.git

diff --git a/compiler/rustc_codegen_ssa/src/mir/block.rs b/compiler/rustc_codegen_ssa/src/mir/block.rs
index 29b7c9b0a88..0802067cde6 100644
--- a/compiler/rustc_codegen_ssa/src/mir/block.rs
+++ b/compiler/rustc_codegen_ssa/src/mir/block.rs
@@ -17,6 +17,7 @@
 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 @@ fn codegen_switchint_terminator(
         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 @@ fn codegen_switchint_terminator(
                 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(),