An unexplained 10% speed-up with gcc-4.8
Hello All, When I started using gcc-4.8.1 I was glad to observe a substantial speed-up of about 10% in my code, as compared with gcc-4.7.3. Usually, switching to newer compilers has a relatively minor effect and definitely not a 10% speed-up. Was there anything significant in gcc-4.8.1 which may explain this dramatic improvement? My code is C++98 which is compiled with profile-driven optimizations, with -O2. My target is generic 32 bit Intel architecture. The result is run on Intel Xeon. The application is CPU intensive. Michael
Re: An unexplained 10% speed-up with gcc-4.8
On 23/12/13 12:40, Michael Veksler wrote: Hello All, When I started using gcc-4.8.1 I was glad to observe a substantial speed-up of about 10% in my code, as compared with gcc-4.7.3. Usually, switching to newer compilers has a relatively minor effect and definitely not a 10% speed-up. Was there anything significant in gcc-4.8.1 which may explain this dramatic improvement? My code is C++98 which is compiled with profile-driven optimizations, with -O2. My target is generic 32 bit Intel architecture. The result is run on Intel Xeon. The application is CPU intensive. After some more testing, I found out that there is about 12% improvement even when comparing two executables compiled without profile-driven optimization. Unfortunately, the vast improvement is observed only for x86, not for x86-64. The speed-up on x86-64 is "only" 2-3%. Michael.
error in converting macro IS_EXPR_CODE_CLASS() to function
IS_EXPR_CODE_CLASS() is called at 18 places within gcc subdirectory,
and except for expr_check(), tree_block(), tree_set_block() all the
other callers pass argument of type enum tree_code_class to
IS_EXPR_CODE_CLASS().
These four callers (expr_check is overloaded) assign value of
TREE_CODE_CLASS() to variable of type char const, and then pass it as
argument to IS_EXPR_CODE_CLASS()
For example: tree_block():
tree
tree_block (tree t)
{
char const c = TREE_CODE_CLASS (TREE_CODE (t));
if (IS_EXPR_CODE_CLASS (c))
return LOCATION_BLOCK (t->exp.locus);
gcc_unreachable ();
return NULL;
}
Should type of "c" be changed to const enum tree_code_class instead
(similarly in other callers) ? Also, TREE_CODE_CLASS()'s value is of
type enum tree_code_class.
This gave a compile-error: invalid conversion from ‘char’ to ‘tree_code_class’
when i changed the macro IS_EXPR_CODE_CLASS() to the following function:
static inline bool
IS_EXPR_CODE_CLASS(enum tree_code_class code_class)
{
return (code_class >= tcc_reference) && (code_class <= tcc_expression);
}
Thanks and Regards,
Prathamesh
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