• Nick Desaulniers's avatar
    ARM: 9256/1: NWFPE: avoid compiler-generated __aeabi_uldivmod · 32200220
    Nick Desaulniers authored
    clang-15's ability to elide loops completely became more aggressive when
    it can deduce how a variable is being updated in a loop. Counting down
    one variable by an increment of another can be replaced by a modulo
    operation.
    
    For 64b variables on 32b ARM EABI targets, this can result in the
    compiler generating calls to __aeabi_uldivmod, which it does for a do
    while loop in float64_rem().
    
    For the kernel, we'd generally prefer that developers not open code 64b
    division via binary / operators and instead use the more explicit
    helpers from div64.h. On arm-linux-gnuabi targets, failure to do so can
    result in linkage failures due to undefined references to
    __aeabi_uldivmod().
    
    While developers can avoid open coding divisions on 64b variables, the
    compiler doesn't know that the Linux kernel has a partial implementation
    of a compiler runtime (--rtlib) to enforce this convention.
    
    It's also undecidable for the compiler whether the code in question
    would be faster to execute the loop vs elide it and do the 64b division.
    
    While I actively avoid using the internal -mllvm command line flags, I
    think we get better code than using barrier() here, which will force
    reloads+spills in the loop for all toolchains.
    
    Link: https://github.com/ClangBuiltLinux/linux/issues/1666Reported-by: default avatarNathan Chancellor <nathan@kernel.org>
    Reviewed-by: default avatarArnd Bergmann <arnd@arndb.de>
    Signed-off-by: default avatarNick Desaulniers <ndesaulniers@google.com>
    Tested-by: default avatarNathan Chancellor <nathan@kernel.org>
    Cc: stable@vger.kernel.org
    Signed-off-by: default avatarRussell King (Oracle) <rmk+kernel@armlinux.org.uk>
    32200220
Makefile 521 Bytes