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Bug in llvm compiler



Hello,
I'm writing optimization of string.c. The point is that functions
there run strlen() and then in general discard the returned
information by running strcpy(). Consider this:

http://www.opensource.apple.com/source/Libc/Libc-997.1.1/string/strcpy.c

Of course there are different implementations that will not run
strlen() second time:

http://www.opensource.apple.com/source/Libc/Libc-262/i386/gen/strcpy.c

but still - utilizing the information allows for optimizations.

I think I've encountered a bug in Apple's llvm. Changing 11 line in
the patch, from "if( l < 8 ) {" to "if( l < 0 ) {" causes the script
to run for 2.5 seconds. Changing it to "if( l < 1 ) {" restores
running time of 2 seconds. Looking at generated assembly shows that
"if( l < 0 ) {" is treated as "if( 0 )" and only the memcpy() part is
emitted. That's fine, but why does that optimized version run slower
having in mind that "if( l < 1 ) {" is impossible condition (every
string has at least 1 byte). The problem doesn't reproduce on FreeBSD
10.1 and Ubuntu 12.10, running times are equal there. I thought I will
show the asm source, maybe someone will find something interesting in
it. In general this seems a bug that should be maybe considered as Zsh
uses memcpy() in various places (also: google
"-Wno-builtin-memcpy-chk-size"). As for string.c, I will provide
memcpy's implementation taken from glibc or other library.

The compiler is:
# gcc --version
Configured with:
--prefix=/Applications/Xcode.app/Contents/Developer/usr
--with-gxx-include-dir=/usr/include/c++/4.2.1
Apple LLVM version 5.0 (clang-500.2.79) (based on LLVM 3.3svn)
Target: x86_64-apple-darwin13.1.0
Thread model: posix

Best regards,
Sebastian Gniazdowski

Attachment: copymemory.patch
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Attachment: out_if_smaller_t0.asm
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Attachment: out_if_smaller_t1.asm
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Attachment: opttest3.zsh
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