I haven't tested it with glDrawArraysIndirect, but for example with glDrawElementsBaseVertex(mode,count,type,indices,basevertex):
- on AMD series 2 to 6, the built-in shader variable gl_VertexID contains basevertex+index
- on AMD series 7, the built-in shader variable gl_VertexID contains just the index
- on Nvidia, the built-in shader variable gl_VertexID contains just the index
Today I tested the 4.3 preview driver 13.15.100.1, and there it seems to be even worse: previously I could at least detect & treat it, but here with some shaders on series 7 it is the index, on others (usually more complex ones) it's basevertex+index ...