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It could be that the destination of the copy is
a spill location, so we have to check the type
of i.to first.
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It would be nicer to have some kind of subtyping where a
l variable can be used in place of a w variable but it is
more complications than a truncation insutruction.
In some cases, it might be less efficient than the above
solution by requiring two registers instead of one.
(I.e when both the truncated variable and the long variable
live together.)
We have to see what are the usage patterns.
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In emit, I worked a little to make sure that framesz works
when we change the size of the svec array (if we need more
alignments).
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It turned out to be not so useful to have a MEM
type for references. Instead I used an OAddr
instruction that translates simply to a lea.
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This is a hack implementation, ideally I would like
something cleaner. The problem is that regalloc thinks
an instruction is dead code because it's not aware that
it writes to eflags.
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It seems that the MEM reference type is
meaningless in too many positions.
Because of this, it is unclear if we
should keep it or just introduce a
OAddr instruction that only accepts
slots.
Regardless of the above, the spilling
module needs to use the new slot_()
function, also, the emit function needs
to fetch the size of the stack frame
from the slot[] array.
The naming is still very transitional,
here is a list of all bogus names I can
think of:
- SLOT()
- Tmp.spill
- slot_
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I've been septic since I introduced it, this commit
proves that it costs more than it helps. I've also fixed
a bad bug in rega() where I alloc'ed the wrong size for
internal arrays. Enums now have names so I can use them
to cast in gdb to get the name corresponding to a constant.
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This new machine-independent mechanism might not be general
enough in the long term but, now, it provides a flexible way
to inform the spiller about the maximum number of arguments
of an instruction that can be spill locations.
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The substraction contrained the register allocator
to allocate a different register for the result and
the second operand, now, we use a neg trick to compile
it down. The machinery that was setup is, regardless,
interesting and will have to be used for floating
point computations (division).
The first bug in rega made broke the explicited loop
invariant: we were using register allocation unavailable
information from other blocks. It's still unclear
how we got wrong results from that considering mappings
are all 0-initialized.
The second bug is a stupid one, one sizeof operator was
missing from a memcpy...
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There are two things I overlooked so far.
1. Binary instructions like cmp that do not have a result
in registers need the size suffix sometimes, for example
when comparing a spill location with a constant.
2. The register allocator needs to be adapted to support the
comparison instruction: it is not possible to compare two
spill locations without using a register.
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The commutativity information only makes sense for
arithmetic expressions. To account for that, I introduced
a new tri-valued boolean type B3. Memory operations, for
example, will receive an undefined commutativity trit.
The code emitter was buggy when rega emitted instructions
like 'rax = add 1, rax', this is now fixed using the
commutativity information (we rewrite it in 'rax = add
rax, 1').
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