RISC-V: fix FP calling conventions

This is a follow-up to e81d015e3.

In the RISC-V ABI, FP arguments to functions are passed in integer registers
(or pairs of integer registers) in two cases:
1- the FP argument is a variadic argument
2- the FP argument is a fixed argument but all 8 FP registers reserved for
   parameter passing have been used already.

The previous implementation handled only case 1, with some problems.

This commit implements both 1 and 2.  To this end, 8 extra FP
caller-save registers are used to hold the values of the FP arguments
that must be passed in integer registers.  Fixup code moves these FP
registers to integer registers / register pairs.  Symmetrically, at
function entry, the integer registers / register pairs are moved back
to the FP registers.

8 extra FP registers is enough because there are only 8 integer
registers used for parameter passing, so at most 8 FP arguments may
need to be moved to integer registers.

1 files changed, 1 insertions, 1 deletions

diff --git a/test/regression/Results/varargs2 b/test/regression/Results/varargs2
index 050b6331..9e77da1b 100644
--- a/test/regression/Results/varargs2
+++ b/test/regression/Results/varargs2
@@ -10,5 +10,5 @@ Twice: -1 1.23
 With va_copy: -1 1.23
 With va_copy: -1 1.23
 With extra args: x & Hello, world! & 42 & 123456789012345 & 3.141592654 & 2.718281746
-With extra FP args: 123456789012345 & 3.141592654 & 2.718281746
+With extra FP args: 3.141592654 & 2.718281746 & 1 & 2 & 3 & 4 & 5 & 6 & 7 & 8 & 42
 va_list compatibility: x & Hello, world! & 42 & 123456789012345 & 3.141592654 & 2.718281746