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We may well treat all rets as
non-escaping since stack slots
are destroyed upon funcion
return.
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Symbols are a useful abstraction
that occurs in both Con and Alias.
In this patch they get their own
struct. This new struct packages
a symbol name and a type; the type
tells us where the symbol name
must be interpreted (currently, in
gobal memory or in thread-local
storage).
The refactor fixed a bug in
addcon(), proving the value of
packaging symbol names with their
type.
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We will be using it in the new
coalesce() pass.
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The asserts (a->type == ABot) made it
impossible to run fillalias() multiple
times. We now reset the Alias.type field
of all temps before starting.
Getting rid of the asserts would have
been another option.
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Stack slots may have padding
bytes, and if we want to have
precise liveness information
it's important that we are able
to tell them apart.
This patch extends fillalias()
to remember for every slot
what bytes were ever assigned.
In case the slot address does
not escape we know that only
these bytes matter.
To save space, we only store
this information if the slot
size is less than or equal to
NBit.
The Alias struct was reworked
a bit to save some space. I am
still not very satisfied with
its layout though.
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We had the invariant that it'd
always be a temporary.
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Thanks to Michael Forney for spotting this
oversight and providing the test case.
Note: because esc() leaves ABot unchanged,
the assertion "a->type == ABot" on line 122
remains valid.
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Symbols in the source file are still limited in
length because the rest of the code assumes that
strings always fit in NString bytes.
Regardless, there is already a benefit because
comparing/copying symbol names does not require
using strcmp()/strcpy() anymore.
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The previous code was buggy. It would put a stack
pointer on the heap when handling "add $foo, 42".
The new code is more straightforward and hopefully
more correct. Only temporaries with a "stack"
alias class will have a slot pointer.
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When a temporary marked local is escaping,
the whole slot must be marked as such. To
solve this, Alias now holds a pointer to
the alias information of the slot. For
simplicity of the code, this pointer is
always valid and fetching ->type out of it
is meaningful.
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