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Identifiers with double-underscore in them are reserved in SkSL.

This isn't a reservation in SKSL. Section 3.6 of the GLSL spec has this restriction. "In
addition, all identifiers containing two consecutive underscores (__) are reserved as possible future keywords."

SKSL is leaving the identifier as-is when transpiling to GLSL. This may lead to a situation where a valid SKSL shader doesn't generate valid GLSL. Perhaps this is also a Skia bug. But yeah, fixing it here sounds good as we'll side-step this issue in ImpellerC.

EDIT: Oof, what a mess. The 4.60 spec clarifies: "Defining or undefining such a name in a shader does not itself result in an error, but may result in unintended behaviors that stem from having multiple definitions of the same name." I think the Android emulator is incorrect in treating this as an error.

Ahh. Thanks for clarifying and bringing it up with the Skia team @chinmaygarde.

This is an oversight in SkSL. We should reject this user code. I'll get in a fix.

Filed as https://bugs.chromium.org/p/skia/issues/detail?id=13740

I have an SkSL fix on deck at http://review.skia.org/580587

SkSL will continue to allow identifiers with multiple underscores. The GLSL backend will perform mangling as needed to make sure that we emit valid GLSL identifiers even if the original identifier contained __.

It should be safe for Impeller to ignore this issue going forward, as SkSL will paper over it for you.

The SkSL mangling fix has been submitted.