[RISCV] Hard float ABI support
The RISC-V hard float calling convention requires the frontend to:
- Detect cases where, once "flattened", a struct can be passed using
int+fp or fp+fp registers under the hard float ABI and coerce to the
- Track usage of GPRs and FPRs in order to gate the above, and to
determine when signext/zeroext attributes must be added to integer
This patch attempts to do this in compliance with the documented ABI,
and uses ABIArgInfo::CoerceAndExpand in order to do this. @rjmccall, as
author of that code I've tagged you as reviewer for initial feedback on
Note that a previous version of the ABI indicated that when passing an
int+fp struct using a GPR+FPR, the int would need to be sign or
zero-extended appropriately. GCC never did this and the ABI was changed,
which makes life easier as ABIArgInfo::CoerceAndExpand can't currently
handle sign/zero-extension attributes.
Re-landed after backing out 366450 due to missed hunks.
Differential Revision: https://reviews.llvm.org/D60456