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Mark inactive Fortran calls - #3194

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Mark inactive Fortran calls#3194
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@joewallwork joewallwork commented Sep 8, 2026

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Refinement of #3168

This PR marks a few Fortran-related things as inactive in the activity analysis.

vchuravy and others added 2 commits September 8, 2026 15:38
@vchuravy @claude @joewallwork
Treat LLVM flang's I/O runtime and type queries as inactive
4bb636b 
KnownInactiveFunctionsStartingWith already lists "f90io", classic flang's I/O
runtime. LLVM flang names the same thing _FortranAio*, which was not covered,
so differentiating any Fortran routine that prints -- DVODE's xerrwd, for
instance -- failed with "No forward mode derivative found".

Also add _FortranAClassIs and _FortranATrim, a type query and character
handling; neither moves floating point data.

_FortranAAssign is deliberately not included. It copies data which may include
reals, so declaring it inactive would silently drop a derivative; it needs a
rule that assigns the shadow too.

On a whole-program Fortran module differentiating DVODE, with loose types, this
takes the error count from 17 to 1, the remainder being that _FortranAAssign.
check-enzyme and check-typeanalysis are unchanged.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
@joewallwork
Rework comments for clarity
b52ae59
@joewallwork joewallwork self-assigned this Sep 8, 2026
@joewallwork joewallwork added the fortran Related to Enzyme's Fortran bindings label Sep 8, 2026
@vchuravy

vchuravy commented Sep 9, 2026

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Some Claude review:

  • The _FortranAio prefix also matches flang's READ entry points (_FortranAioInputReal32/64, InputComplex*, InputDescriptor, InputUnformattedBlock), which overwrite user floating-point memory, so marking them inactive leaves the destination shadow un-zeroed and turns a loud failure into a silently wrong derivative.

  • Adding these names to the inactive lists is not sufficient for reverse mode, because the constant-fallback path calls EnzymeLogic::CreateNoFree on the declaration-only callee, which hard-errors since no _FortranA* name is in its NoFrees list.

  • The new comment's justification that "the comparison above is unsigned, so an entry at the any-offset index -1 is skipped" holds only for a 64-bit size_t, making the block's behavior platform-dependent and documenting an accident as intent.

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