>> Please give an overview of what APEX is and why LTO currently
>> cannot handle it.
>>
>> Richard.
> Luis, is the problem just that we need consistent builtin numbering (which
> requires special care with dynamically enabled builtins but should still be
> possible) or is there more to it?
> --
> Regards
> Robin
Hi Richard, Robin,
APEX (ARC Processor EXtension) is a mechanism for integrating custom
instructions into GCC without requiring users to have detailed knowledge of
compiler internals. It allows users to define custom instructions via C pragmas
and assembler directives.
A function prototype is used to represent a custom instruction, and a pragma
binds it to instruction metadata (e.g., mnemonic, opcode), effectively making it
behave like a builtin:
```c
int foo_func (int, int);
#pragma intrinsic (foo_func, "foo", 7, XD)
int main (void)
{
return foo_func (1, 2);
}
```
which generate:
```asm
.extInstruction foo,7,XD
main:
li a5,2
li a0,1
foo a0,a0,a5 # APEX instruction
ret
```
The issue with LTO is that APEX intrinsics are registered dynamically during
front-end processing via pragmas. This registration state is not preserved
through LTO.
By the time LTO reads the serialized GIMPLE back in, the original pragma
processing phase has already completed, so the backend no longer has any record
of the intrinsic definition (mnemonic, opcode, operand constraints). As a
result, the call cannot be resolved against any known intrinsic entry, leading
to an ICE during validation.
In response to Robin’s question: it is a bit more than just builtin numbering.
It is not only about having a stable identifier across LTO, but also about
preserving and restoring the intrinsic registration itself. Even with
consistent
numbering, the backend still lacks the metadata required to reconstruct the
intrinsic after LTO.
For additional background on the APEX design and overall compilation flow
(not specific to LTO), I presented it at GNU Cauldron 2025:
https://www.youtube.com/watch?v=jHfxkAN3Qtw
Regards,
Luis
GF Confidential – Need to know
