Date: Fri, 1 Jul 2011 18:51:51 +0200 From: Roman Divacky <rdivacky@freebsd.org> To: Marcel Moolenaar <marcel@xcllnt.net> Cc: svn-src-projects@freebsd.org, Marcel Moolenaar <marcel@freebsd.org>, src-committers@freebsd.org Subject: Re: svn commit: r223705 - projects/llvm-ia64/lib/clang/libllvmjit Message-ID: <20110701165151.GA6877@freebsd.org> In-Reply-To: <00211D6B-F882-43C1-9D93-5ED2D72C5132@xcllnt.net> References: <201107010329.p613Tn8s071270@svn.freebsd.org> <20110701084224.GA43291@freebsd.org> <00211D6B-F882-43C1-9D93-5ED2D72C5132@xcllnt.net>
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> The following open items are on my mind:
>
> 1. On ia64, function prologues allocate a register frame that has
> enough (stacked) registers for local scratch registers and
> outgoing function arguments. This means that I need to know
> (after register allocation) how many (unique) scratch registers
> are in use and what the largest number of arguments that need
> to be passed in registers to children (the max being 8). Without
> this information the compiler is forced to allocate the maximum
> size (which is 96 stacked registers, of which 8 are outgoing).
> This obviously eats into the register stack and probably causes
> runtime failures on deep call chains.
I recommend you to do this little experiment (on amd64 or so):
pes ~$ cat test.c
void foo(int i) {
printf("foo %i\n", i);
}
int main(int agc, char **argv)
{
printf("hello world\n");
for (unsigned i = 0; i < 5; ++i)
foo(i);
}
pes ~$ clang -emit-llvm -c test.c test.c:2:3: warning: implicitly declaring C library function 'printf' with type 'int (const char *, ...)'
printf("foo %i\n", i);
^
test.c:2:3: note: please include the header <stdio.h> or explicitly provide a declaration for 'printf'
1 warning generated.
pes ~$ llc -debug test.o
the llc -debug will show you a lot of interesting information on how llvm handles this code.
Among others you'll find there:
# Machine code for function foo:
Frame Objects:
fi#0: size=4, align=4, at location [SP+8]
Function Live Ins: %EDI in %vreg0
I believe this is what you asked.
> 2. [C++] vtable entries are not function pointers like on other
> architectures. They are function descriptors. I think Nathan
> said that PowerPC64 also have function descriptors. Anyway,
> Duraid Madina (the author of the original ia64 backend) said
> that support for this was missing from LLVM. I don't know if
> this has changed in the mean time or whether I need to go
> down into the bowels of LLVM and add support for this.
I would forget about C++ for now.
roman
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