Re: [LLVMdev] inline asm semantics: output constraint width smaller than input
From: H. Peter Anvin
Date: Tue Jan 27 2009 - 16:26:00 EST
Duncan Sands wrote:
Hi,
If yes then this doesnt look all that bad or invasive at first sight (if
the put_user() workaround can be expressed in a cleaner way), but in any
case it would be nice to hear an LLVM person's opinion about roughly when
this is going to be solved in LLVM itself.
one thing that seems to be clear to everyone except me is... what are the
semantics supposed to be? [My understanding is that what is being discussed
is when you have an asm with a register as input and output, but with integer
types of different width for the input and output, but I saw some mention of
struct types in this thread...]. Presumably this is something obvious, but
it would be good to have someone spell it out in small words that even someone
like me can understand :)
I don't know about struct types, but the situation I'm talking about is
assembly statements of the form:
asm("foo" : "=r" (bar) : "0" (baz));
Here, "bar" and "baz" are constrained to be in the same hardware
register (from the "0" constraint in "baz"). The types of "bar" and
"baz" are otherwise unrelated.
I assume the difficulty here comes from how this needs to be handled
from the point of view of the register allocator. If both types fit
inside a single allocatable hardware register, the issue is trivial;
"bar" and "baz" form a single logical register for the purpose of
register allocation.
However, things get a bit ugly in the case of different widths that
affect individually scheduled registers, like 32- and 64-bit types on a
32-bit machine. Consider the case above where "bar" is a 64-bit type
and "baz" is a 32-bit type, then you functionally have, at least on x86:
uint64_t tmp = bar;
asm("foo" : "+r" (tmp));
baz = (uint32_t)tmp;
One could possibly argue that the latter case should be
"baz = (uint32_t)(tmp >> 32);" on a bigendian machine... since this is a
gcc syntax it probably should be "whatever gcc does" in that case, as
opposed to what might make sense.
(I'm afraid I don't have a bigendian box readily available at the
moment, so I can't test it out to see what gcc does. I have a powerpc
machine, but it's at home and turned off.)
-hpa
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