Don't relax a branch to have different input constraints.

When relaxing a branch, restrict the set of candidate encodings to those which
have the same input constraints as the original encoding choice. This prevents
situations where relaxation prefers a non-REX-prefixed encoding over a REX
prefixed one because the end of the instruction can be one byte closer to the
destination, in a situation where the encoding needs to be REX-prefixed
because of one of the operand registers.

This also makes the Context class perform encoding verification after
relaxation, to catch similar problems in the future.

Fixes #256.

lib/cretonne/src/context.rs

@@ -131,6 +131,20 @@ impl Context {
         }
     }
 
+    /// Run the locations verifier on the function.
+    pub fn verify_locations<'a>(&self, isa: &TargetIsa) -> verifier::Result {
+        verifier::verify_locations(isa, &self.func, None)
+    }
+
+    /// Run the locations verifier only if the `enable_verifier` setting is true.
+    pub fn verify_locations_if<'a>(&self, isa: &TargetIsa) -> CtonResult {
+        if isa.flags().enable_verifier() {
+            self.verify_locations(isa).map_err(Into::into)
+        } else {
+            Ok(())
+        }
+    }
+
     /// Perform pre-legalization rewrites on the function.
     pub fn preopt(&mut self, isa: &TargetIsa) -> CtonResult {
         do_preopt(&mut self.func);
@@ -212,13 +226,16 @@ impl Context {
     /// Insert prologue and epilogues after computing the stack frame layout.
     pub fn prologue_epilogue(&mut self, isa: &TargetIsa) -> CtonResult {
         isa.prologue_epilogue(&mut self.func)?;
-        self.verify_if(isa)
+        self.verify_if(isa)?;
+        self.verify_locations_if(isa)?;
+        Ok(())
     }
 
     /// Run the branch relaxation pass and return the final code size.
     pub fn relax_branches(&mut self, isa: &TargetIsa) -> Result<CodeOffset, CtonError> {
         let code_size = relax_branches(&mut self.func, isa)?;
         self.verify_if(isa)?;
+        self.verify_locations_if(isa)?;
 
         Ok(code_size)
     }