[msl-out] Correctly construct out-of-bounds values for all types.

src/back/msl/keywords.rs

@@ -210,4 +210,6 @@ pub const RESERVED: &[&str] = &[
     "M_2_SQRTPI",
     "M_SQRT2",
     "M_SQRT1_2",
+    // Naga utilities
+    "DefaultConstructible",
 ];

src/back/msl/writer.rs

@@ -822,7 +822,7 @@ impl<W: Write> Writer<W> {
                 {
                     write!(self.out, " ? ")?;
                     self.put_access_chain(expr_handle, policy, context)?;
-                    write!(self.out, " : 0")?;
+                    write!(self.out, " : DefaultConstructible()")?;
 
                     if !is_scoped {
                         write!(self.out, ")")?;
@@ -1529,7 +1529,7 @@ impl<W: Write> Writer<W> {
         {
             write!(self.out, " ? ")?;
             self.put_unchecked_load(pointer, policy, context)?;
-            write!(self.out, " : 0")?;
+            write!(self.out, " : DefaultConstructible()")?;
 
             if !is_scoped {
                 write!(self.out, ")")?;
@@ -2154,6 +2154,13 @@ impl<W: Write> Writer<W> {
         writeln!(self.out, "#include <simd/simd.h>")?;
         writeln!(self.out)?;
 
+        if options
+            .bounds_check_policies
+            .contains(index::BoundsCheckPolicy::ReadZeroSkipWrite)
+        {
+            self.put_default_constructible()?;
+        }
+
         {
             let mut indices = vec![];
             for (handle, var) in module.global_variables.iter() {
@@ -2181,6 +2188,28 @@ impl<W: Write> Writer<W> {
         self.write_functions(module, info, options, pipeline_options)
     }
 
+    /// Write the definition for the `DefaultConstructible` class.
+    ///
+    /// The [`ReadZeroSkipWrite`] bounds check policy requires us to be able to
+    /// produce 'zero' values for any type, including structs, arrays, and so
+    /// on. We could do this by emitting default constructor applications, but
+    /// that would entail printing the name of the type, which is more trouble
+    /// than you'd think. Instead, we just construct this magic C++14 class that
+    /// can be converted to any type that can be default constructed, using
+    /// template parameter inference to detect which type is needed, so we don't
+    /// have to figure out the name.
+    ///
+    /// [`ReadZeroSkipWrite`]: index::BoundsCheckPolicy::ReadZeroSkipWrite
+    fn put_default_constructible(&mut self) -> BackendResult {
+        writeln!(self.out, "struct DefaultConstructible {{")?;
+        writeln!(self.out, "    template<typename T>")?;
+        writeln!(self.out, "    operator T() && {{")?;
+        writeln!(self.out, "        return T {{}};")?;
+        writeln!(self.out, "    }}")?;
+        writeln!(self.out, "}};")?;
+        Ok(())
+    }
+
     fn write_type_defs(&mut self, module: &crate::Module) -> BackendResult {
         for (handle, ty) in module.types.iter() {
             if !ty.needs_alias() {

src/proc/index.rs

@@ -148,6 +148,11 @@ impl BoundsCheckPolicies {
             _ => self.index,
         }
     }
+
+    /// Return `true` if any of `self`'s policies are `policy`.
+    pub fn contains(&self, policy: BoundsCheckPolicy) -> bool {
+        self.index == policy || self.buffer == policy || self.image == policy
+    }
 }
 
 /// An index that may be statically known, or may need to be computed at runtime.
@@ -204,9 +209,7 @@ pub fn find_checked_indexes(
     let mut guarded_indices = BitSet::new();
 
     // Don't bother scanning if we never need `ReadZeroSkipWrite`.
-    if policies.index == BoundsCheckPolicy::ReadZeroSkipWrite
-        || policies.buffer == BoundsCheckPolicy::ReadZeroSkipWrite
-    {
+    if policies.contains(BoundsCheckPolicy::ReadZeroSkipWrite) {
         for (_handle, expr) in function.expressions.iter() {
             // There's no need to handle `AccessIndex` expressions, as their
             // indices never need to be cached.

tests/out/msl/bounds-check-zero.msl

@@ -2,6 +2,12 @@
 #include <metal_stdlib>
 #include <simd/simd.h>
 
+struct DefaultConstructible {
+    template<typename T>
+    operator T() && {
+        return T {};
+    }
+};
 struct _mslBufferSizes {
     metal::uint size0;
 };
@@ -23,7 +29,7 @@ float index_array(
     device Globals& globals,
     constant _mslBufferSizes& _buffer_sizes
 ) {
-    float _e4 = metal::uint(i) < 10 ? globals.a.inner[i] : 0;
+    float _e4 = metal::uint(i) < 10 ? globals.a.inner[i] : DefaultConstructible();
     return _e4;
 }
 
@@ -32,7 +38,7 @@ float index_dynamic_array(
     device Globals& globals,
     constant _mslBufferSizes& _buffer_sizes
 ) {
-    float _e4 = metal::uint(i_1) < 1 + (_buffer_sizes.size0 - 112 - 4) / 4 ? globals.d[i_1] : 0;
+    float _e4 = metal::uint(i_1) < 1 + (_buffer_sizes.size0 - 112 - 4) / 4 ? globals.d[i_1] : DefaultConstructible();
     return _e4;
 }
 
@@ -41,7 +47,7 @@ float index_vector(
     device Globals& globals,
     constant _mslBufferSizes& _buffer_sizes
 ) {
-    float _e4 = metal::uint(i_2) < 4 ? globals.v[i_2] : 0;
+    float _e4 = metal::uint(i_2) < 4 ? globals.v[i_2] : DefaultConstructible();
     return _e4;
 }
 
@@ -49,7 +55,7 @@ float index_vector_by_value(
     metal::float4 v,
     int i_3
 ) {
-    return metal::uint(i_3) < 4 ? v[i_3] : 0;
+    return metal::uint(i_3) < 4 ? v[i_3] : DefaultConstructible();
 }
 
 metal::float4 index_matrix(
@@ -57,7 +63,7 @@ metal::float4 index_matrix(
     device Globals& globals,
     constant _mslBufferSizes& _buffer_sizes
 ) {
-    metal::float4 _e4 = metal::uint(i_4) < 3 ? globals.m[i_4] : 0;
+    metal::float4 _e4 = metal::uint(i_4) < 3 ? globals.m[i_4] : DefaultConstructible();
     return _e4;
 }
 
@@ -67,7 +73,7 @@ float index_twice(
     device Globals& globals,
     constant _mslBufferSizes& _buffer_sizes
 ) {
-    float _e6 = metal::uint(j) < 4 && metal::uint(i_5) < 3 ? globals.m[i_5][j] : 0;
+    float _e6 = metal::uint(j) < 4 && metal::uint(i_5) < 3 ? globals.m[i_5][j] : DefaultConstructible();
     return _e6;
 }
 
@@ -77,7 +83,7 @@ float index_expensive(
     constant _mslBufferSizes& _buffer_sizes
 ) {
     int _e9 = static_cast<int>(metal::sin(static_cast<float>(i_6) / 100.0) * 100.0);
-    float _e11 = metal::uint(_e9) < 10 ? globals.a.inner[_e9] : 0;
+    float _e11 = metal::uint(_e9) < 10 ? globals.a.inner[_e9] : DefaultConstructible();
     return _e11;
 }
 

tests/out/msl/policy-mix.msl

@@ -2,6 +2,12 @@
 #include <metal_stdlib>
 #include <simd/simd.h>
 
+struct DefaultConstructible {
+    template<typename T>
+    operator T() && {
+        return T {};
+    }
+};
 struct type_1 {
     metal::float4 inner[10];
 };