Currently, the diff tool matches types bottom up, so on every
instruction it expects to know if its operands are already matched or
not. With cyclical references, it cannot know that. Type matching
would need significant rework to be able to support such a use case; for
example, it may need to maintain a set of plausable matches between type
pointers that are forward-referenced, and potentially back track when
later the types turn out to be incompatible.
In this change, OpTypeForwardPointer is supported in the more common and
trivial case. Firstly, forwarded type pointers are only matched if they
have they have the same storage class and point to the same type opcode:
- In the presence of debug info, matching is done only if the names are
unique in both src and dst.
- In the absence of debug info, matching is done only if there is only
one possible matching.
Fixes: #4754
spirv-diff is a new tool that produces diff-style output comparing two
SPIR-V modules. The instructions between the src and dst modules are
matched as best as the tool can, and output is produced (in src
id-space) that shows which instructions are removed in src, added in dst
or modified between them. The order of instructions are not retained.
Matching instructions between two SPIR-V modules is not trivial, and
thus a number of heuristics are applied in this tool. In particular,
without debug information, it's hard to match functions as they can be
reordered. As such, this tool is primarily useful to produce the diff
of two SPIR-V modules derived from the same source.
This tool can be useful in a number of scenarios:
- Compare the SPIR-V before and after modifying a shader
- Compare the SPIR-V produced from a shader before and after compiler
codegen changes.
- Compare the SPIR-V produced from a shader before and after some
transformation or optimization.
- Compare the SPIR-V produced from a shader with different compilers.
