On 2/18/25 7:30 PM, Jin Ma wrote:
I apologize for not explaining things more clearly. I also discovered that
the issue is caused by CSE. I think that during the substitution process,
CSE recognized the syntax of if_then_else and concluded that the expressions
in the "then" and "else" branches are equivalent, resulting in both yielding
(reg/v:RVVMF2SF 140 [ vreg_memory ]):
(minus:RVVMF2SF (reg/v:RVVMF2SF 140 [ vreg_memory ])
(float_extend:RVVMF2SF (vec_duplicate:RVVMF4HF (const_double:HF 0.0
[0x0.0p+0]))))
is considered equivalent to:
(reg/v:RVVMF2SF 140 [ vreg_memory ])
Clearly, there wasn’t a deeper consideration of the fact that float_extend
requires
a rounding mode(frm). Therefore, I attempted to use UNSPEC in the pattern to
inform
CSE that we have a rounding mode.
Right. It worked, but there's a deeper issue here.
As I mentioned before, this may not be a good solution, as it risks missing
other
optimization opportunities. As you pointed out, we need a more general approach
to fix it. Unfortunately, while I’m still trying to find a solution, I currently
don't have any other good ideas.
Changing the rounding modes isn't common, but it's not unheard of. My
suspicion is that we need to expose the rounding mode assignment earlier
(at RTL generation time).
That may not work well with the current optimization of FRM, but I think
early exposure is the only viable path forward in my mind. Depending on
the depth of the problems it may not be something we can fix in the
gcc-15 space.
You might experiment with emitting the FRM assignment in the
insn_expander class in the risc-v backend. This code:
/* Add rounding mode operand. */
if (m_insn_flags & FRM_DYN_P)
add_rounding_mode_operand (FRM_DYN);
else if (m_insn_flags & FRM_RUP_P)
add_rounding_mode_operand (FRM_RUP);
else if (m_insn_flags & FRM_RDN_P)
add_rounding_mode_operand (FRM_RDN);
else if (m_insn_flags & FRM_RMM_P)
add_rounding_mode_operand (FRM_RMM);
else if (m_insn_flags & FRM_RNE_P)
add_rounding_mode_operand (FRM_RNE);
else if (m_insn_flags & VXRM_RNU_P)
add_rounding_mode_operand (VXRM_RNU);
else if (m_insn_flags & VXRM_RDN_P)
add_rounding_mode_operand (VXRM_RDN);
For anything other than FRM_DYN_P emit the appropriate insn to set FRM.
This may generate poor code in the presence of explicit rounding modes,
but I think something along these lines is ultimately going to be needed.
jeff
