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The arm64 ABI needs to know precisely what
floating point types are being used, so we
need to store that information.
I also made typ[] a dynamic array.
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This big diff does multiple changes to allow
the addition of new targets to qbe. The
changes are listed below in decreasing order
of impact.
1. Add a new Target structure.
To add support for a given target, one has to
implement all the members of the Target
structure. All the source files where changed
to use this interface where needed.
2. Single out amd64-specific code.
In this commit, the amd64 target T_amd64_sysv
is the only target available, it is implemented
in the amd64/ directory. All the non-static
items in this directory are prefixed with either
amd64_ or amd64_sysv (for items that are
specific to the System V ABI).
3. Centralize Ops information.
There is now a file 'ops.h' that must be used to
store all the available operations together with
their metadata. The various targets will only
select what they need; but it is beneficial that
there is only *one* place to change to add a new
instruction.
One good side effect of this change is that any
operation 'xyz' in the IL now as a corresponding
'Oxyz' in the code.
4. Misc fixes.
One notable change is that instruction selection
now generates generic comparison operations and
the lowering to the target's comparisons is done
in the emitter.
GAS directives for data are the same for many
targets, so data emission was extracted in a
file 'gas.c'.
5. Modularize the Makefile.
The Makefile now has a list of C files that
are target-independent (SRC), and one list
of C files per target. Each target can also
use its own 'all.h' header (for example to
define registers).
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The size of a union is the size of the largest
element aligned with the largest alignment.
For example, the size of the following union is
16, not 13 (as returned before this patch).
union {
char c[13];
int i;
};
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The spec says that numbers can be arbitrarily
big, and only the last 64 bits will be taken
into consideration. Calling sscanf does not
implement this, so I wrote an ad-hoc function.
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Compiling languages with closures often requires passing
an extra environment parameter to the called function.
One solution is to use a convention, and reserve, say,
the first argument for that purpose. However, that
makes binding to C a little less smooth.
Alternatively, QBE now provides a way to remain fully
ABI compatible with C by having a "hidden" environment
argument (marked with the keyword 'env'). Calling a
function expecting an environment from C will make the
contents of the environment undefined, but the normal
arguments will be passed without alteration. Conversely,
calling a C function like it is a closure by passing
it an environemnt will work smoothly.
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This change is backward compatible, calls to
"variadic" functions (like printf) must now be
annotated (with ...).
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This happens to be needed for C. The standard mandates that a return
value is used if the caller uses it. Surprisingly, if the return
"value" is not used, the callee can use "return;".
A better solution is to add an "undef" value and return it, "undef"
would also have other use cases for compiling C.
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The type sizes are important to get right because
the ABI relies on them when it emits memory blits
to pass/return structs.
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AFL found this bug.
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I thought it would be harder (and maybe it is).
My fear was that a call must be always followed by
a parallel move from machine registers (this is an
assumption in both spill and rega). This however
remains true, because the ABI code generates a
dummy "copy RAX" by accident!
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