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Date: Tue, 27 Jan 2009 17:56:00 -0800
From: "H. Peter Anvin" <hpa@...or.com>
To: Kyle Moffett <kyle@...fetthome.net>
CC: Duncan Sands <baldrick@...e.fr>, llvmdev@...uiuc.edu,
Ingo Molnar <mingo@...e.hu>,
Török Edwin <edwintorok@...il.com>,
Thomas Gleixner <tglx@...utronix.de>,
Linux Kernel <linux-kernel@...r.kernel.org>
Subject: Re: [LLVMdev] inline asm semantics: output constraint width smaller
than input
Kyle Moffett wrote:
>
> Actually, PPC64 boxes basically don't care... the usable GPRs are all
> either 32-bit (for PPC32) or 64-bit (for PPC64), the <=32-bit
> instructions are identical across both, they just
> truncate/sign-extend/etc based on the lower 32-bits of the register.
> Also, you would only do a right-shift if you were going all the way
> out to memory as 64-bit and all the way back in as 32-bit... within a
> single register it's kept coherent.
>
Think about a 64-bit integer on ppc32. It will by necessity kept in two
registers. On gcc I believe it will always be a consecutive pair of
registers (AFAIK that's a hard-coded assumption in gcc, with the result
that gcc has a nonstandard internal register numbering for x86 since the
commonly used dx:ax pair is actually registers 2:0 in the hardware
numbering.)
> Structs are basically irrelevant for inline ASM as you can't pass a
> struct to one... you can only pass the *address* of a struct, which is
> always pointer-sized.
Right, of course.
> I think that really the only sane solution (which is hopefully what
> GCC does) for integer types is to use a register the same size as the
> larger of the two integers. Then you copy the value to/from the
> smaller register (or just mask it on PPC64-alike architectures) before
> or after the inline ASM.
Pretty much. Then you can do conventional copy propagation and
elimination after expanding subregisters to get rid of the extra ops in
the common case.
-hpa
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