Mark eligible `INTENT(IN)` dummy data objects with a `fir.read_only`
argument attribute during lowering, and teach `FunctionAttr` to
translate the marker to LLVM `readonly` for reference arguments when
optimizing for speed.
LLVM FunctionAttrs can infer `readonly` later in the pipeline, but this
happens too late for IPSCCP function specialization.
`FunctionSpecializer::promoteConstantStackValues` requires
`onlyReadsMemory()` before promoting constant by-reference scalar
arguments. Providing `readonly` satisfies this check and allows these
arguments to be promoted, enabling function specialization, including in
non-LTO builds.
For direct-data arguments, restrict propagation to by-reference
non-character intrinsic scalars. Ordinary arrays and derived-type
arguments are excluded because compiler-generated copy-out may write
back through a forwarded dummy argument. LLVM `readonly` does not permit
such writes, even when the stored value is unchanged.
`INTENT(IN)` `POINTER` and `ALLOCATABLE` dummies are supported with
shallow descriptor semantics: `readonly` protects the descriptor
storage, but does not imply that data addressed by the descriptor is
read-only. In particular, a `POINTER` target may still be modified.
Keep `readonly` handling independent from `noalias` and `nocapture`.
`TARGET` arguments may receive `readonly`, but must not receive
`noalias` or `nocapture`. Apply `readonly` to `BIND(C)` definitions
while leaving external C declarations unannotated. `VALUE`,
`ASYNCHRONOUS`, and `VOLATILE` dummy arguments remain excluded.
In our measurements, the enabled function specialization gives a 1.5x
speedup on 548.exchange2 on NVIDIA Neoverse V2 in a non-LTO
configuration.
Update existing lowering tests to account for the new `fir.read_only`
argument attribute, and add focused coverage:
* `dummy-argument-readonly.f90` checks marker emission for supported
scalar and descriptor arguments, excluded cases, and shallow pointer
semantics.
* `function-attrs-readonly.fir` checks translation from `fir.read_only`
to LLVM `readonly`, including `TARGET`, `BIND(C)`, and descriptor
references.
* `function-attr-readonly.f90` checks the optimization-level pipeline
gate and verifies that `readonly` reaches final LLVM IR.
* `call-copy-in-out.f90` checks that ordinary array copy-out cases
remain unmarked and that pointer-target copy-out is compatible with
shallow descriptor `readonly`.
Sergey Shcherbinin
a70234b00d