http://llvm.org/bugs/show_bug.cgi?id=17603
Bug ID: 17603
Summary: x86: optimize byte+shift load to unaligned load
Product: libraries
Version: trunk
Hardware: PC
OS: All
Status: NEW
Severity: enhancement
Priority: P
Component: Backend: X86
Assignee: [email protected]
Reporter: [email protected]
CC: [email protected]
Classification: Unclassified
Created attachment 11382
--> http://llvm.org/bugs/attachment.cgi?id=11382&action=edit
Sample C file with safe-load-version/ unaligned-one + disasm
Hello,
In wireshark we have bunch of safe (both alignment-safe, and endianess) macros
to read uint16_t/uint32_t/uint64_t from byte array.
They looks like this:
#define pntohs(p) ((guint16) \
((guint16)*((const guint8 *)(p)+0)<<8| \
(guint16)*((const guint8 *)(p)+1)<<0))
#define pletohs(p) ((guint16) \
((guint16)*((const guint8 *)(p)+1)<<8| \
(guint16)*((const guint8 *)(p)+0)<<0))
#define pntohl(p) ((guint32)*((const guint8 *)(p)+0)<<24| \
(guint32)*((const guint8 *)(p)+1)<<16| \
(guint32)*((const guint8 *)(p)+2)<<8| \
(guint32)*((const guint8 *)(p)+3)<<0)
It'd be great if clang could detect such operations and change it on x86/AMD64
to:
#define pntohs(p) ((uint16_t) __builtin_bswap16(*(uint16_t *) p)))
#define plethos(p) ((uint16_t) (*(uint16_t *) p))
#define pntohl(p) ((uint32_t) __builtin_bswap32(*(uint32_t *) p)))
Generally the idea is the same like in Linux Kernel (config symbol:
HAVE_EFFICIENT_UNALIGNED_ACCESS selected on x86/ppc) or ffmpeg (#ifdef
HAVE_FAST_UNALIGNED) to do unaligned access on CPUs which can do it fast (i.e.:
faster than byte&shift load).
We could also do it in wireshark. but I like more idea of writting portable
code, where low-level optimization like above are done on compiler-level, not
in code.
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