GCC provides two interfaces for accessing the Hardware Transactional Memory (HTM) instructions available on some of the PowerPC family of prcoessors (eg, POWER8). The two interfaces come in a low level interface, consisting of built-in functions specific to PowerPC and a higher level interface consisting of inline functions that are common between PowerPC and S/390.
The following low level built-in functions are available with -mhtm or -mcpu=CPU where CPU is `power8' or later. They all generate the machine instruction that is part of the name.
The HTM builtins (with the exception of __builtin_tbegin) return
the full 4-bit condition register value set by their associated hardware
instruction. The header file htmintrin.h defines some macros that can
be used to decipher the return value. The __builtin_tbegin builtin
returns a simple true or false value depending on whether a transaction was
successfully started or not. The arguments of the builtins match exactly the
type and order of the associated hardware instruction's operands, except for
the __builtin_tcheck builtin, which does not take any input arguments.
Refer to the ISA manual for a description of each instruction's operands.
unsigned int __builtin_tbegin (unsigned int)
unsigned int __builtin_tend (unsigned int)
unsigned int __builtin_tabort (unsigned int)
unsigned int __builtin_tabortdc (unsigned int, unsigned int, unsigned int)
unsigned int __builtin_tabortdci (unsigned int, unsigned int, int)
unsigned int __builtin_tabortwc (unsigned int, unsigned int, unsigned int)
unsigned int __builtin_tabortwci (unsigned int, unsigned int, int)
unsigned int __builtin_tcheck (void)
unsigned int __builtin_treclaim (unsigned int)
unsigned int __builtin_trechkpt (void)
unsigned int __builtin_tsr (unsigned int)
In addition to the above HTM built-ins, we have added built-ins for some common extended mnemonics of the HTM instructions:
unsigned int __builtin_tendall (void)
unsigned int __builtin_tresume (void)
unsigned int __builtin_tsuspend (void)
The following set of built-in functions are available to gain access to the HTM specific special purpose registers.
unsigned long __builtin_get_texasr (void)
unsigned long __builtin_get_texasru (void)
unsigned long __builtin_get_tfhar (void)
unsigned long __builtin_get_tfiar (void)
void __builtin_set_texasr (unsigned long);
void __builtin_set_texasru (unsigned long);
void __builtin_set_tfhar (unsigned long);
void __builtin_set_tfiar (unsigned long);
Example usage of these low level built-in functions may look like:
#include <htmintrin.h>
int num_retries = 10;
while (1)
{
if (__builtin_tbegin (0))
{
/* Transaction State Initiated. */
if (is_locked (lock))
__builtin_tabort (0);
... transaction code...
__builtin_tend (0);
break;
}
else
{
/* Transaction State Failed. Use locks if the transaction
failure is "persistent" or we've tried too many times. */
if (num_retries-- <= 0
|| _TEXASRU_FAILURE_PERSISTENT (__builtin_get_texasru ()))
{
acquire_lock (lock);
... non transactional fallback path...
release_lock (lock);
break;
}
}
}
One final built-in function has been added that returns the value of
the 2-bit Transaction State field of the Machine Status Register (MSR)
as stored in CR0.
unsigned long __builtin_ttest (void)
This built-in can be used to determine the current transaction state using the following code example:
#include <htmintrin.h>
unsigned char tx_state = _HTM_STATE (__builtin_ttest ());
if (tx_state == _HTM_TRANSACTIONAL)
{
/* Code to use in transactional state. */
}
else if (tx_state == _HTM_NONTRANSACTIONAL)
{
/* Code to use in non-transactional state. */
}
else if (tx_state == _HTM_SUSPENDED)
{
/* Code to use in transaction suspended state. */
}
The following high level HTM interface is made available by including
<htmxlintrin.h> and using -mhtm or -mcpu=CPU
where CPU is `power8' or later. This interface is common between PowerPC
and S/390, allowing users to write one HTM source implementation that
can be compiled and executed on either system.
long __TM_simple_begin (void)
long __TM_begin (void* const TM_buff)
long __TM_end (void)
void __TM_abort (void)
void __TM_named_abort (unsigned char const code)
void __TM_resume (void)
void __TM_suspend (void)
long __TM_is_user_abort (void* const TM_buff)
long __TM_is_named_user_abort (void* const TM_buff, unsigned char *code)
long __TM_is_illegal (void* const TM_buff)
long __TM_is_footprint_exceeded (void* const TM_buff)
long __TM_nesting_depth (void* const TM_buff)
long __TM_is_nested_too_deep(void* const TM_buff)
long __TM_is_conflict(void* const TM_buff)
long __TM_is_failure_persistent(void* const TM_buff)
long __TM_failure_address(void* const TM_buff)
long long __TM_failure_code(void* const TM_buff)
Using these common set of HTM inline functions, we can create a more portable version of the HTM example in the previous section that will work on either PowerPC or S/390:
#include <htmxlintrin.h>
int num_retries = 10;
TM_buff_type TM_buff;
while (1)
{
if (__TM_begin (TM_buff) == _HTM_TBEGIN_STARTED)
{
/* Transaction State Initiated. */
if (is_locked (lock))
__TM_abort ();
... transaction code...
__TM_end ();
break;
}
else
{
/* Transaction State Failed. Use locks if the transaction
failure is "persistent" or we've tried too many times. */
if (num_retries-- <= 0
|| __TM_is_failure_persistent (TM_buff))
{
acquire_lock (lock);
... non transactional fallback path...
release_lock (lock);
break;
}
}
}