Auto merge of rust-lang#120650 - clubby789:switchint-const, r=<try>

Use `br` instead of a conditional when switching on a constant boolean

r? `@ghost`

Author: bors

compiler/rustc_codegen_ssa/src/mir/block.rs

@@ -319,6 +319,13 @@ impl<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>> FunctionCx<'a, 'tcx, Bx> {
         targets: &SwitchTargets,
     ) {
         let discr = self.codegen_operand(bx, discr);
+        // If our discr is a constant we can branch directly
+        if let Some(const_discr) = bx.const_to_opt_u128(discr.immediate(), true) {
+            let target = targets.target_for_value(const_discr);
+            bx.br(helper.llbb_with_cleanup(self, target));
+            return;
+        };
+
         let switch_ty = discr.layout.ty;
         let mut target_iter = targets.iter();
         if target_iter.len() == 1 {
@@ -329,6 +336,16 @@ impl<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>> FunctionCx<'a, 'tcx, Bx> {
             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.
+                /*let discr = discr.immediate();
+                match (bx.const_to_opt_uint(discr), test_value) {
+                    (Some(0), 0) => bx.br(lltrue),
+                    (Some(1), 0) => bx.br(llfalse),
+                    (Some(0), 1) => bx.br(llfalse),
+                    (Some(1), 1) => bx.br(lltrue),
+                    (None, 0) => bx.cond_br(discr, llfalse, lltrue),
+                    (None, 1) => bx.cond_br(discr, lltrue, llfalse),
+                    _ => bug!(),
+                }*/
                 match test_value {
                     0 => bx.cond_br(discr.immediate(), llfalse, lltrue),
                     1 => bx.cond_br(discr.immediate(), lltrue, llfalse),