Sass Diff

SASS Diffs

Use this when asked to check for SASS changes between commits, branches or a local changeset.

Goal

Detect relevant changes in generated CUDA machine code (i.e. SASS) while filtering noise from addresses, symbols, metadata, etc.

Any non-trivial change must be detected.

Inputs to establish

• Compilation target under test
• The CUDA SM architectures to compile for. Try to detect this from the code and offer the user a list of suggestions.

The user must confirm or provide this list.

• Baseline source (e.g. the previous commit/branch or the current commit without the changes in the working copy).
• Comparison source (e.g. the current commit/branch or the current commit with the changes in the working copy).
• Whether a SASS (default) or PTX diff is requested.

Disassembly listing generation

• Compile both, the baseline and comparison source, with the same compiler flags and options.

When not specified otherwise, lookup the options from compile_commands.json

or the current build system (i.e. CMake files).

Make sure the CUDA SM architectures (CMAKE_CUDA_ARCHITECTURES) are set to the user-provided or approved list.

• Dump the disassembly from the binaries produced in the previous set using cuobjdump -sass or cuobjdump -ptx.

Comparison rules (what matters)

Ignore as trivial:

• Register renaming with identical instruction sequence and operands.
• Pure label renumbering or reordering of identical basic blocks.
• Formatting-only differences or reordered symbol tables.
• Changes to symbol names (global function names)

Reporting

• If any non-trivial change was detected, report the top 5 regions where a non-trivial change was detected,

including the name of the kernel they appeared in.

• Provide a short summary of the diff type,

including opcode changes, memory access size/cache policy changes, control-flow changes, register-count changes,

spills/local memory, shared memory, and occupancy-relevant resource deltas.

• Explicitly state if only noise was detected.
• If you are not sure if the differences are impactful, show it and ask the user for guidance.
• Keep the disassembly dumps available and tell the user where they can find them.