gen_settings: dont try to display a Preset descriptor in Flags (#241)

* gen_settings: dont try to display a Preset descriptor in Flags

Trying to display a preset doesnt make sense, and before this commit it
does not display anything meaningful - the printout just says e.g.
"haswell =\n".

The offset byte a preset descriptor isnt a valid offset into the
flag bytes, it is actually an offset into the PRESETS table. It will
cause a panic when the offset is out of bounds for the flag bytes,
which happens in the intel isa as of this commit.

* intel settings: test that display impl doesnt panic

lib/cretonne/meta/gen_settings.py

@@ -218,6 +218,7 @@ def gen_display(sgrp, fmt):
                 '}'):
             fmt.line('writeln!(f, "[{}]")?;'.format(sgrp.name))
             with fmt.indented('for d in &DESCRIPTORS {', '}'):
+                with fmt.indented('if !d.detail.is_preset() {', '}'):
                     fmt.line('write!(f, "{} = ", d.name)?;')
                     fmt.line(
                             'TEMPLATE.format_toml_value(d.detail,' +

lib/cretonne/src/isa/intel/settings.rs

@@ -30,4 +30,23 @@ mod tests {
         assert_eq!(f2.has_sse41(), true);
         assert_eq!(f2.has_bmi1(), true);
     }
+    #[test]
+    fn display_presets() {
+        // Spot check that the flags Display impl does not cause a panic
+        let shared = settings::Flags::new(&settings::builder());
+
+        let b1 = builder();
+        let f1 = Flags::new(&shared, &b1);
+        let _ = format!("{}", f1);
+
+        let mut b2 = builder();
+        b2.enable("nehalem").unwrap();
+        let f2 = Flags::new(&shared, &b1);
+        let _ = format!("{}", f2);
+
+        let mut b3 = builder();
+        b3.enable("haswell").unwrap();
+        let f3 = Flags::new(&shared, &b1);
+        let _ = format!("{}", f3);
+    }
 }

lib/cretonne/src/settings.rs

@@ -297,6 +297,17 @@ pub mod detail {
         /// The `Descriptor::offset` field refers to the `PRESETS` table.
         Preset,
     }
+
+    impl Detail {
+        /// Check if a detail is a Detail::Preset. Useful because the Descriptor
+        /// offset field has a different meaning when the detail is a preset.
+        pub fn is_preset(&self) -> bool {
+            match self {
+                &Detail::Preset => true,
+                _ => false,
+            }
+        }
+    }
 }
 
 // Include code generated by `meta/gen_settings.py`. This file contains a public `Flags` struct