Commit 0d482775cc58a086009e63b2dc288ed29904529e | zig

author	Matthew Lugg <mlugg@mlugg.co.uk> 2025-07-29 11:18:48+01:00
committer	Matthew Lugg <mlugg@mlugg.co.uk> 2025-07-29 15:52:19+01:00
log	0d482775cc58a086009e63b2dc288ed29904529e
tree	125c49fcfeefe874f5d144e7683db7230797f238
parent	d0bc5efba4979471f092a533c20ad04f9e730dec
signature	Commit is signed but in an unrecognized format.

Sema: don't rely on Liveness

We're currently experimenting with backends which effectively do their
own liveness analysis, so this old trick of mine isn't necessarily valid
anymore. However, we can fix that trivially: just make the "nop"
instruction we jam into here have the right type. That way, the leftover
field/element pointer instructions are perfectly valid, but still
unused.

1 files changed, 7 insertions(+), 8 deletions(-)

src
- Sema.zig

src/Sema.zig+7-8

...	...	@@ -4113,14 +4113,13 @@ fn finishResolveComptimeKnownAllocPtr(
4113	4113	// We're almost done - we have the resolved comptime value. We just need to
4114	4114	// eliminate the now-dead runtime instructions.
4115	4115
4116		// We will rewrite the AIR to eliminate the alloc and all stores to it.
4117		// This will cause instructions deriving field pointers etc of the alloc to
4118		// become invalid, however, since we are removing all stores to those pointers,
4119		// they will be eliminated by Liveness before they reach codegen.
4120
4121		// The specifics of this instruction aren't really important: we just want
4122		// Liveness to elide it.
4123		const nop_inst: Air.Inst = .{ .tag = .bitcast, .data = .{ .ty_op = .{ .ty = .u8_type, .operand = .zero_u8 } } };
	4116	// This instruction has type `alloc_ty`, meaning we can rewrite the `alloc` AIR instruction to
	4117	// this one to drop the side effect. We also need to rewrite the stores; we'll turn them to this
	4118	// too because it doesn't really matter what they become.
	4119	const nop_inst: Air.Inst = .{ .tag = .bitcast, .data = .{ .ty_op = .{
	4120	.ty = .fromIntern(alloc_ty.toIntern()),
	4121	.operand = .zero_usize,
	4122	} } };
4124	4123
4125	4124	sema.air_instructions.set(@intFromEnum(alloc_inst), nop_inst);
4126	4125	for (comptime_info.stores.items(.inst)) \|store_inst\| {