Messages in this thread | | | Date | Tue, 27 Jan 2009 13:25:18 -0800 | From | "H. Peter Anvin" <> | Subject | Re: [LLVMdev] inline asm semantics: output constraint width smaller than input |
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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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