Commit b341b2d0 authored by Simon Marlow's avatar Simon Marlow

add comments

parent fcd3864f
......@@ -35,13 +35,55 @@ import GLAEXTS
-- -----------------------------------------------------------------------------
-- The mini-inliner
-- This pass inlines assignments to temporaries that are used just
-- once in the very next statement only. Generalising this would be
-- quite difficult (have to take into account aliasing of memory
-- writes, and so on), but at the moment it catches a number of useful
-- cases and lets the code generator generate much better code.
This pass inlines assignments to temporaries that are used just
once. It works as follows:
- count uses of each temporary
- for each temporary that occurs just once:
- attempt to push it forward to the statement that uses it
- only push forward past assignments to other temporaries
(assumes that temporaries are single-assignment)
- if we reach the statement that uses it, inline the rhs
and delete the original assignment.
Possible generalisations: here is an example from factorial
_smi = R2;
if (_smi != 0) goto cmK;
R1 = R3;
jump I64[Sp];
_smn = _smi * R3;
R2 = _smi + (-1);
R3 = _smn;
jump Fac_zdwfac_info;
We want to inline _smi and _smn. To inline _smn:
- we must be able to push forward past assignments to global regs.
We can do this if the rhs of the assignment we are pushing
forward doesn't refer to the global reg being assigned to; easy
to test.
To inline _smi:
- It is a trivial replacement, reg for reg, but it occurs more than
- We can inline trivial assignments even if the temporary occurs
more than once, as long as we don't eliminate the original assignment
(this doesn't help much on its own).
- We need to be able to propagate the assignment forward through jumps;
if we did this, we would find that it can be inlined safely in all
its occurrences.
It catches many useful cases, but could be generalised in
-- several ways.
-- NB. This assumes that temporaries are single-assignment.
cmmMiniInline :: [CmmBasicBlock] -> [CmmBasicBlock]
cmmMiniInline blocks = map do_inline blocks
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