`sret` arguments are always going to reside in the stack/`alloca`
address space, which makes the current formulation where their AS is
derived from the pointee somewhat quaint. This patch ensures that `sret`
ends up pointing to the `alloca` AS in IR function signatures, and also
guards agains trying to pass a casted `alloca`d pointer to a `sret` arg,
which can happen for most languages, when compiled for targets that have
a non-zero `alloca` AS (e.g. AMDGCN) / map `LangAS::default` to a
non-zero value (SPIR-V). A target could still choose to do something
different here, by e.g. overriding `classifyReturnType` behaviour.
In a broader sense, this patch extends non-aliased indirect args to also
carry an AS, which leads to changing the `getIndirect()` interface. At
the moment we're only using this for (indirect) returns, but it allows
for future handling of indirect args themselves. We default to using the
AllocaAS as that matches what Clang is currently doing, however if, in
the future, a target would opt for e.g. placing indirect returns in some
other storage, with another AS, this will require revisiting.
---------
Co-authored-by: Matt Arsenault <arsenm2@gmail.com>
Co-authored-by: Matt Arsenault <Matthew.Arsenault@amd.com>
For a default visibility external linkage definition, dso_local is set for ELF
-fno-pic/-fpie and COFF and Mach-O. Since default clang -cc1 for ELF is similar
to -fpic ("PIC Level" is not set), this nuance causes unneeded binary format differences.
To make emitted IR similar, ELF -cc1 -fpic will default to -fno-semantic-interposition,
which sets dso_local for default visibility external linkage definitions.
To make this flip smooth and enable future (dso_local as definition default),
this patch replaces (function) `define ` with `define{{.*}} `,
(variable/constant/alias) `= ` with `={{.*}} `, or inserts appropriate `{{.*}} `.
Summary:
Clang -fpic defaults to -fno-semantic-interposition (GCC -fpic defaults
to -fsemantic-interposition).
Users need to specify -fsemantic-interposition to get semantic
interposition behavior.
Semantic interposition is currently a best-effort feature. There may
still be some cases where it is not handled well.
Reviewers: peter.smith, rnk, serge-sans-paille, sfertile, jfb, jdoerfert
Subscribers: dschuff, jyknight, dylanmckay, nemanjai, jvesely, kbarton, fedor.sergeev, asb, rbar, johnrusso, simoncook, sabuasal, niosHD, jrtc27, zzheng, edward-jones, atanasyan, rogfer01, MartinMosbeck, brucehoult, the_o, arphaman, PkmX, jocewei, jsji, Jim, lenary, s.egerton, pzheng, sameer.abuasal, apazos, luismarques, cfe-commits
Tags: #clang
Differential Revision: https://reviews.llvm.org/D73865
