[RISCV] Add custom CC_RISCV calling convention and improved call support

The TableGen-based calling convention definitions are inflexible, while
writing a function to implement the calling convention is very
straight-forward, and allows difficult cases to be handled more easily. With
this patch adds support for:
* Passing large scalars according to the RV32I calling convention
* Byval arguments
* Passing values on the stack when the argument registers are exhausted

The custom CC_RISCV calling convention is also used for returns.

This patch also documents the ABI lowering that a language frontend is 
expected to perform. I would like to work to simplify these requirements over 
time, but this will require further discussion within the LLVM community.

We add PendingArgFlags CCState, as a companion to PendingLocs.

The PendingLocs vector is used by a number of backends to handle arguments 
that are split during legalisation. However CCValAssign doesn't keep track of 
the original argument alignment. Therefore, add a PendingArgFlags vector which 
can be used to keep track of the ISD::ArgFlagsTy for every value added to 
PendingLocs.

Differential Revision: https://reviews.llvm.org/D39898

llvm-svn: 320359

1 files changed, 2 insertions, 14 deletions

diff --git a/llvm/lib/Target/RISCV/RISCVCallingConv.td b/llvm/lib/Target/RISCV/RISCVCallingConv.td
index 0b7a523424c..d2b17c64c9c 100644
--- a/llvm/lib/Target/RISCV/RISCVCallingConv.td
+++ b/llvm/lib/Target/RISCV/RISCVCallingConv.td
@@ -11,20 +11,8 @@
 //
 //===----------------------------------------------------------------------===//
 
-// RISCV 32-bit C return-value convention.
-def RetCC_RISCV32 : CallingConv<[CCIfType<[i32], CCAssignToReg<[X10, X11]>>]>;
-
-// RISCV 32-bit C Calling convention.
-def CC_RISCV32 : CallingConv<[
-  // Promote i8/i16 args to i32
-  CCIfType<[ i8, i16 ], CCPromoteToType<i32>>,
-
-  // All arguments get passed in integer registers if there is space.
-  CCIfType<[i32], CCAssignToReg<[ X10, X11, X12, X13, X14, X15, X16, X17]>>,
-
-  // Could be assigned to the stack in 8-byte aligned units, but unsupported
-  CCAssignToStack<8, 8>
-]>;
+// The RISC-V calling convention is handled with custom code in
+// RISCVISelLowering.cpp (CC_RISCV).
 
 def CSR : CalleeSavedRegs<(add X1, X3, X4, X8, X9, (sequence "X%u", 18, 27))>;