Hello everyone,
I am Saswat Padhi, a final year undergraduate at IIT Bombay, India.
With help from the GCC Resource Center here, I am currently investing
possible improvements to GCC Machine Descriptions, with a goal of
making them concise and composable.
One of the improvements we were looking at is, reducing the C code
fragments within the define_expand constructs in the machine
descriptions. In some of the define_expands we observed that the C
code fragments implement the same checks which can equivalently be
achieved through the use of constraints. So, we thought that
introducing constraints-validation at the expander level could be
useful. An example of change we are proposing, is illustrated below:
(define_expand "movsi"
[(set (match_operand:SI 0 "nonimmediate_operand" "")
(match_operand:SI 1 "general_operand" "")
)]
""
{ if(GET_CODE(operands[0])==MEM && GET_CODE(operands[1])!=REG)
{ if(can_create_pseudo_p())
{ operands[1]=force_reg(SImode,operands[1]); }
}})
With the expander handling the constraints, the above define_expand
can be written as:
(define_expand "movsi"
[(set (match_operand:SI 0 "nonimmediate_operand" "=r,m,r,r")
(match_operand:SI 1 "general_operand" "r,r,i,m")
)]
""
)
This concise define_expand expression would achieve the desired effect
because when the first operand matches the second constraint
alternative [m], the second operand would be force_reg into the second
alternative [r]. All other cases are also explicitly mentioned as
alternative constraints.
As a first step towards this, I prepared this report
(https://docs.google.com/document/d/1Yb5b3-r4YfxEL0omvAqqGFGIYLLeMenn-EHQvSHgKUw/edit)
which is a survey of the define_expand expressions in i386 and MIPS
machine descriptions. But as the report explains, the results of the
survey do not appear in our favor. We see very few define_expands
where we can actually replace some parts of the C code by introducing
constraint validation in the expander.
I seek feedback from the community regarding:
1. Is the idea is worth pursuing? In other words, would we be able to
reduce significant amount of C code from the GCC MDs by introducing
constraint validation at expander?
2. If answer to (1) is yes, do the parameters mentioned in the report
make sense? We use 3 parameters to estimate whether we would be able
to simplify a define_expand by removing at least a part of the C code.
3. If the answer to (2) is yes, why don't we see any gain in i386 and
MIPS MDs? Then are there other cases where the gain would be
significant?
Thank you in advance for your time and help :-)
