Re:Lock elision in the GNU C library
by callan on Wednesday February 06, 2013 @10:27AM
(#5176803)
HLE is a pretty nifty implementation (note: I work for Intel) and an interesting use of CISC.
IA instructions can optionally contain a prefix, such as LOCK, address and operand override (use 16, 32, or 64 bits), and repeat (REP). For example, in Linux, a single "REP STOS" (repeat store string) instruction is used to zero-out the kernel's BSS region. REP can also be added to instructions that don't use it, and the CPU will simply ignore it--this is defined behavior.
For HLE, using REP in front of certain instructions on a spinlock will begin or end a single memory transaction without actually writing the spinlock value. Of course, on older CPUs, the REP will be ignored and the spinlock read/write will occur. On CPUs that support HLE: if another memory agent accesses the spinlock, the transaction will be canceled.
IA instructions can optionally contain a prefix, such as LOCK, address and operand override (use 16, 32, or 64 bits), and repeat (REP). For example, in Linux, a single "REP STOS" (repeat store string) instruction is used to zero-out the kernel's BSS region. REP can also be added to instructions that don't use it, and the CPU will simply ignore it--this is defined behavior.
For HLE, using REP in front of certain instructions on a spinlock will begin or end a single memory transaction without actually writing the spinlock value. Of course, on older CPUs, the REP will be ignored and the spinlock read/write will occur. On CPUs that support HLE: if another memory agent accesses the spinlock, the transaction will be canceled.