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The ARM version of __div64_32() encapsulates a call to __do_div64 with non-standard argument passing. In particular, __n is a 64-bit input argument assigned to r0-r1 and __rem is an output argument sharing half of that r0-r1 register pair. With __n being an input argument, the compiler is in its right to presume that r0-r1 would still hold the value of __n past the inline assembly statement. Normally, the compiler would have assigned non overlapping registers to __n and __rem if the value for __n is needed again. However, here we enforce our own register assignment and gcc fails to notice the conflict. In practice this doesn't cause any problem as __n is considered dead after the asm statement and *n is overwritten. However this is not always guaranteed and clang rightfully complains. Let's fix it properly by making __n into an input-output variable. This makes it clear that those registers representing __n have been modified. Then we can extract __rem as the high part of __n with plain C code. This asm constraint "abuse" was likely relied upon back when gcc didn't handle 64-bit values optimally. Turns out that gcc is now able to optimize things and produces the same code with this patch applied. Reported-by: Antony Yu <swpenim@gmail.com> Signed-off-by: Nicolas Pitre <nico@fluxnic.net> Reviewed-by: Ard Biesheuvel <ardb@kernel.org> Tested-by: Ard Biesheuvel <ardb@kernel.org> Signed-off-by: Russell King <rmk+kernel@armlinux.org.uk> |
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Linux kernel
============
There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
Documentation/admin-guide/README.rst first.
In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``. The formatted documentation can also be read online at:
https://www.kernel.org/doc/html/latest/
There are various text files in the Documentation/ subdirectory,
several of them using the Restructured Text markup notation.
Please read the Documentation/process/changes.rst file, as it contains the
requirements for building and running the kernel, and information about
the problems which may result by upgrading your kernel.