GLSL ES2 behavior is explicitly undefined if an out-param is never
written to: "If a function does not write to an out parameter, the value
of the actual parameter is undefined when the function returns."
We do see divergence here in practice: SkVM's behavior (the parameter is
left alone) differs from my GPU's behavior (the parameter is zeroed
out).
SkSL will now report an error if an out parameter is never assigned-to.
There is no control flow analysis performed, so we will not report
cases where the out parameter is assigned-to on some paths but not
others. (Technically the return-on-all-paths logic could be adapted
for this, but it would be a fair amount of work.)
Structs are currently exempt from the rule because custom mesh
specifications require an `out` parameter for a Varyings struct, even if
your mesh program doesn't need Varyings.
Bug: skia:12867
Change-Id: Ie828d3ce91c2c67e008ae304fdb163ffa88d744c
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/500440
Auto-Submit: John Stiles <johnstiles@google.com>
Reviewed-by: Brian Osman <brianosman@google.com>
Commit-Queue: Brian Osman <brianosman@google.com>
