BLAS: add rules for cblas_?dotc_sub and cblas_?dotu_sub

[vchuravy]

Adds derivative rules for the complex dot products in their cblas _sub form (e.g. cblas_zdotc_sub64_ as used by Julia's BLAS.dotc), whose result is written through a pointer passed as the last argument instead of being returned.

	primal	reverse	forward
dotc_sub	res = Σ conj(x_i)·y_i	dx += conj(dres)·y, dy += dres·x	dres = dotc(dx,y) + dotc(x,dy)
dotu_sub	res = Σ x_i·y_i	dx += dres·y, dy += dres·x	dres = dotu(dx,y) + dotu(x,dy)

All c/z variants and suffixes ("", _, 64_, _64_) are recognized; the adjoints are emitted as cblas_?axpy calls, and overwritten inputs are cached via cblas_?copy.

tblgen changes

• New fpret argument type for a scalar result returned through a pointer. In reverse mode its shadow provides DiffeRet and is reset afterwards (the primal overwrote the result); in forward mode it receives the dual. BlasCalls to *_sub functions get a result alloca appended and their result loaded back, mirroring the existing cublas v2 path.
• New BConj op (complex conjugate of an fp scalar, identity for real types).
• Per-pattern supportsComplex bit: the blanket "complex inputs not yet supported in reverse mode" error now only applies to patterns that have not opted in.
• Complex scalars are passed by pointer under the cblas ABI, so byRefFloat accounts for that; the ABI flags are now emitted by one shared helper for all four generators.
• Small runtime helpers in Utils.cpp for conjugation and for element-aligned load/store of a blas scalar through a pointer that may be passed as an integer (Julia).

Notes

• The result pointer is attributed nocapture but deliberately not writeonly: with writeonly, the unused-value analysis considers a local result buffer (e.g. a Julia Ref/alloca) unneeded and replaces it with a fictitious phi, while the primal call writing to it is kept. That looks like a general gap in calculateUnusedValuesInFunction for calls whose only writes go to unneeded memory; left as is here.
• nocapture is no longer applied to integer-typed vector arguments (invalid IR, hit with Julia-style i64 pointers).

Testing

New lit tests for both functions in forward and reverse mode (test/Enzyme/{ForwardMode,ReverseMode}/blas/cblas_zdot{c,u}_sub64_.ll), including the caching path. Verified locally against LLVM 22; the CHECK lines use regexes tolerant of the typed-pointer / nocapture spelling on older LLVM. Also checked by hand that cblas_cdotc_sub (single precision, 32-bit ints) and the Julia-style form with pointers passed as i64 produce the expected code, and that the existing ddot derivatives are unchanged.

🤖 Generated with Claude Code

https://claude.ai/code/session_01XgNCvikUqowFPsvLWUJqVB