Currently clang does not emit device template variables
instantiated only in host functions, however, nvcc is
able to do that:
https://godbolt.org/z/fneEfferY
This patch fixes this issue by refactoring and extending
the existing mechanism for emitting static device
var ODR-used by host only. Basically clang records
device variables ODR-used by host code and force
them to be emitted in device compilation. The existing
mechanism makes sure these device variables ODR-used
by host code are added to llvm.compiler-used, therefore
they are guaranteed not to be deleted.
It also fixes non-ODR-use of static device variable by host code
causing static device variable to be emitted and registered,
which should not.
Reviewed by: Artem Belevich
Differential Revision: https://reviews.llvm.org/D102237
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 `{{.*}} `.
A static device variable may be accessed in host code through
cudaMemCpyFromSymbol etc. Currently clang does not
emit the static device variable if it is only referenced by
host code, which causes host code to fail at run time.
This patch fixes that.
Differential Revision: https://reviews.llvm.org/D88115
nvcc supports accessing file-scope static device variables in host code by host APIs
like cudaMemcpyToSymbol etc.
CUDA/HIP let users access device variables in host code by shadow variables. In host compilation,
clang emits a shadow variable for each device variable, and calls __*RegisterVariable to
register it in init function. The address of the shadow variable and the device side mangled
name of the device variable is passed to __*RegisterVariable. Runtime looks up the symbol
by name in the device binary to find the address of the device variable.
The problem with static device variables is that they have internal linkage, therefore their
name may be changed by the linker if there are multiple symbols with the same name. Also
they end up as local symbols in the elf file, whereas the runtime only looks up the global symbols.
Another reason for making the static device variables external linkage is that they may be
initialized externally by host code and their final value may be accessed by host code
after kernel execution, therefore they actually have external linkage. Giving them internal
linkage will cause incorrect optimizations on them.
To support accessing static device var in host code for -fno-gpu-rdc mode, change the intnernal
linkage to external linkage. The name does not need change since there is only one TU for
-fno-gpu-rdc mode. Also the externalization is done only if the device static var is referenced
by host code.
Differential Revision: https://reviews.llvm.org/D80858
Yaxun (Sam) Liu