Bug report: Floating conversion from double to simple infinity sets a lot of ones in the mantissa

[flaviens]

Hi there!

I've detected a bug in CVFPU.

Brief bug description

A conversion of +inf from double to simple sets a lot of bits in the mantissa, unexpectedly.
I have found it through cva6.

Example instance

Here is an example RISC-V (rv64imfd) snippet:

  .section ".text.init","ax",@progbits
  .globl _start
  .align 2
_start:

  # Enable the FPU
  li t0, 0x2000
  csrs mstatus, t0
  csrw	fcsr,x0

  la t0, .fdata0
  fld ft0, (t0)

  fcvt.s.d ft1, ft0, rdn

  li t0, 0x18
  fsd ft1, (t0)

  sw x0, 0(x0)

infinite_loop:
  j infinite_loop

.section ".fdata0","ax",@progbits
  .8byte 0x7ff0000000000000

Expected and actual results

We expect ft1= 0xffffffff7f800000.. I verified this with Spike.
However, CVA6 gives ft1= 0xffffffff7f7fffff..

Thanks!
Flavien