AstGen: improved logic for nodeMayNeedMemoryLocation

 * `@as` and `@bitCast` no longer unconditionally return `true` from
   this function; they forward the question to their sub-expression.
 * fix `@splat` incorrectly being marked as needing a memory location
   (this function returns a SIMD vector; it definitely does not want a
   memory location).

Makes AstGen generate slightly nicer ZIR, which in turn generates
slightly nicer AIR, generating slightly nicer machine code in debug
builds.

It also means I can procrastinate implementing the bitcast_result_ptr
ZIR instruction semantic analysis :^)

2 files changed, 37 insertions, 15 deletions

diff --git a/src/AstGen.zig b/src/AstGen.zig
index 847860630a..24a73539b3 100644
--- a/src/AstGen.zig
+++ b/src/AstGen.zig
@@ -8271,17 +8271,31 @@ fn nodeMayNeedMemoryLocation(tree: *const Ast, start_node: Ast.Node.Index) bool
                 }
             },
 
-            .builtin_call,
-            .builtin_call_comma,
-            .builtin_call_two,
-            .builtin_call_two_comma,
-            => {
+            .builtin_call_two, .builtin_call_two_comma => {
+                const builtin_token = main_tokens[node];
+                const builtin_name = tree.tokenSlice(builtin_token);
+                // If the builtin is an invalid name, we don't cause an error here; instead
+                // let it pass, and the error will be "invalid builtin function" later.
+                const builtin_info = BuiltinFn.list.get(builtin_name) orelse return false;
+                switch (builtin_info.needs_mem_loc) {
+                    .never => return false,
+                    .always => return true,
+                    .forward1 => node = node_datas[node].rhs,
+                }
+            },
+
+            .builtin_call, .builtin_call_comma => {
+                const params = tree.extra_data[node_datas[node].lhs..node_datas[node].rhs];
                 const builtin_token = main_tokens[node];
                 const builtin_name = tree.tokenSlice(builtin_token);
                 // If the builtin is an invalid name, we don't cause an error here; instead
                 // let it pass, and the error will be "invalid builtin function" later.
                 const builtin_info = BuiltinFn.list.get(builtin_name) orelse return false;
-                return builtin_info.needs_mem_loc;
+                switch (builtin_info.needs_mem_loc) {
+                    .never => return false,
+                    .always => return true,
+                    .forward1 => node = params[1],
+                }
             },
         }
     }
diff --git a/src/BuiltinFn.zig b/src/BuiltinFn.zig
index 8f23ec86d7..e1f4f5bd16 100644
--- a/src/BuiltinFn.zig
+++ b/src/BuiltinFn.zig
@@ -110,10 +110,19 @@ pub const Tag = enum {
     Vector,
 };
 
+pub const MemLocRequirement = enum {
+    /// The builtin never needs a memory location.
+    never,
+    /// The builtin always needs a memory location.
+    always,
+    /// The builtin forwards the question to argument at index 1.
+    forward1,
+};
+
 tag: Tag,
 
-/// `true` if the builtin call can take advantage of a result location pointer.
-needs_mem_loc: bool = false,
+/// Info about the builtin call's ability to take advantage of a result location pointer.
+needs_mem_loc: MemLocRequirement = .never,
 /// `true` if the builtin call can be the left-hand side of an expression (assigned to).
 allows_lvalue: bool = false,
 /// The number of parameters to this builtin function. `null` means variable number
@@ -148,7 +157,7 @@ pub const list = list: {
             "@as",
             .{
                 .tag = .as,
-                .needs_mem_loc = true,
+                .needs_mem_loc = .forward1,
                 .param_count = 2,
             },
         },
@@ -184,7 +193,7 @@ pub const list = list: {
             "@bitCast",
             .{
                 .tag = .bit_cast,
-                .needs_mem_loc = true,
+                .needs_mem_loc = .forward1,
                 .param_count = 2,
             },
         },
@@ -248,7 +257,7 @@ pub const list = list: {
             "@call",
             .{
                 .tag = .call,
-                .needs_mem_loc = true,
+                .needs_mem_loc = .always,
                 .param_count = 3,
             },
         },
@@ -410,7 +419,7 @@ pub const list = list: {
             "@field",
             .{
                 .tag = .field,
-                .needs_mem_loc = true,
+                .needs_mem_loc = .always,
                 .param_count = 2,
                 .allows_lvalue = true,
             },
@@ -699,7 +708,6 @@ pub const list = list: {
             "@splat",
             .{
                 .tag = .splat,
-                .needs_mem_loc = true,
                 .param_count = 2,
             },
         },
@@ -714,7 +722,7 @@ pub const list = list: {
             "@src",
             .{
                 .tag = .src,
-                .needs_mem_loc = true,
+                .needs_mem_loc = .always,
                 .param_count = 0,
             },
         },
@@ -869,7 +877,7 @@ pub const list = list: {
             "@unionInit",
             .{
                 .tag = .union_init,
-                .needs_mem_loc = true,
+                .needs_mem_loc = .always,
                 .param_count = 3,
             },
         },