Commit fbbf34e563a376ea1654dce827b9194ba7211b3a | zig

author	Matthew Lugg <mlugg@mlugg.co.uk> 2025-02-05 20:33:39+00:00
committer	Matthew Lugg <mlugg@mlugg.co.uk> 2025-02-05 21:17:40+00:00
log	fbbf34e563a376ea1654dce827b9194ba7211b3a
tree	dc95e30f87d14c2ff3b3784df76ed10327f4425a
parent	cac814cf58ca65ffd8081dca6f2f5b26d822ef5d
signature	Commit is signed but in an unrecognized format.

Sema: disable runtime safety checks in comptime blocks

Sometimes we emit runtime instructions in comptime scopes. These
instructions will be discarded, but they allow comptime blocks to
contain intermediate runtime-known values, which is necessary for
expressions like `runtime_array.len` to work.

Since we will always throw away these runtime instructions, including
safety checks is a time waste at best and trips an assertion at worst!

Resolves: #20064

2 files changed, 24 insertions(+), 3 deletions(-)

src
- Sema.zig
test
- behavior
  - eval.zig

src/Sema.zig+13-3

 @@ -504,7 +504,17 @@ pub const Block = struct {
         };
     }
-    pub fn wantSafety(block: *const Block) bool {
+    fn wantSafeTypes(block: *const Block) bool {
+        return block.want_safety orelse switch (block.sema.pt.zcu.optimizeMode()) {
+            .Debug => true,
+            .ReleaseSafe => true,
+            .ReleaseFast => false,
+            .ReleaseSmall => false,
+        };
+    }
+    fn wantSafety(block: *const Block) bool {
+        if (block.isComptime()) return false; // runtime safety checks are pointless in comptime blocks
         return block.want_safety orelse switch (block.sema.pt.zcu.optimizeMode()) {
             .Debug => true,
             .ReleaseSafe => true,
 @@ -3294,7 +3304,7 @@ fn zirUnionDecl(
                 .tagged
             else if (small.layout != .auto)
                 .none
-            else switch (block.wantSafety()) {
+            else switch (block.wantSafeTypes()) {
                 true => .safety,
                 false => .none,
             },
 @@ -22219,7 +22229,7 @@ fn reifyUnion(
                 .tagged
             else if (layout != .auto)
                 .none
-            else switch (block.wantSafety()) {
+            else switch (block.wantSafeTypes()) {
                 true => .safety,
                 false => .none,
             },

test/behavior/eval.zig+11

...	@@ -1751,3 +1751,14 @@ test "comptime labeled block implicit exit" {	...	@@ -1751,3 +1751,14 @@ test "comptime labeled block implicit exit" {
1751	};	1751	};
1752	comptime assert(result == {});	1752	comptime assert(result == {});
1753	}	1753	}
		1754
		1755	test "comptime block has intermediate runtime-known values" {
		1756	const arr: [2]u8 = .{ 1, 2 };
		1757
		1758	var idx: usize = undefined;
		1759	idx = 0;
		1760
		1761	comptime {
		1762	_ = arr[idx];
		1763	}
		1764	}