diff options
Diffstat (limited to 'libm/math_private.h')
| -rw-r--r-- | libm/math_private.h | 44 |
1 files changed, 29 insertions, 15 deletions
diff --git a/libm/math_private.h b/libm/math_private.h index 0601f2c3d..2c5a30ac2 100644 --- a/libm/math_private.h +++ b/libm/math_private.h @@ -188,14 +188,24 @@ extern double __kernel_tan (double,double,int) attribute_hidden; extern int __kernel_rem_pio2 (double*,double*,int,int,int,const int*) attribute_hidden; /* - * math_opt_barrier(x): force expression x to be evaluated and put into - * a floating point register or memory. This macro returns the value. + * math_opt_barrier(x): safely load x, even if it was manipulated + * by non-floationg point operations. This macro returns the value of x. + * This ensures compiler does not (ab)use its knowledge about x value + * and don't optimize future operations. Example: + * float x; + * SET_FLOAT_WORD(x, 0x80000001); // sets a bit pattern + * y = math_opt_barrier(x); // "compiler, do not cheat!" + * y = y * y; // compiler can't optimize, must use real multiply insn * - * math_force_eval(x): force expression x to be evaluated and put into + * math_force_eval(x): force expression x to be evaluated. + * Useful if otherwise compiler may eliminate the expression + * as unused. This macro returns no value. + * Example: "void fn(float f) { f = f * f; }" + * versus "void fn(float f) { f = f * f; math_force_eval(f); }" + * + * Currently, math_force_eval(x) stores x into * a floating point register or memory *of the appropriate size*. - * This forces floating point flags to be set correctly - * (for example, when float value is overflowing, but FPU registers - * are wide enough to "hide" this). + * There is no guarantee this will not change. */ #if defined(__i386__) #define math_opt_barrier(x) ({ \ @@ -205,19 +215,20 @@ extern int __kernel_rem_pio2 (double*,double*,int,int,int,const int*) attribu __x; \ }) #define math_force_eval(x) do { \ - if (sizeof(x) <= sizeof(double)) \ + __typeof(x) __x = (x); \ + if (sizeof(__x) <= sizeof(double)) \ /* "m": store x into a memory location */ \ - __asm __volatile ("" : : "m" (x)); \ + __asm __volatile ("" : : "m" (__x)); \ else /* long double */ \ /* "f": load x into (any) fpreg */ \ - __asm __volatile ("" : : "f" (x)); \ + __asm __volatile ("" : : "f" (__x)); \ } while (0) #endif #if defined(__x86_64__) #define math_opt_barrier(x) ({ \ __typeof(x) __x = (x); \ - if (sizeof(x) <= sizeof(double)) \ + if (sizeof(__x) <= sizeof(double)) \ /* "x": load into XMM SSE register */ \ __asm ("" : "=x" (__x) : "0" (__x)); \ else /* long double */ \ @@ -226,19 +237,22 @@ extern int __kernel_rem_pio2 (double*,double*,int,int,int,const int*) attribu __x; \ }) #define math_force_eval(x) do { \ - if (sizeof(x) <= sizeof(double)) \ + __typeof(x) __x = (x); \ + if (sizeof(__x) <= sizeof(double)) \ /* "x": load into XMM SSE register */ \ - __asm __volatile ("" : : "x" (x)); \ + __asm __volatile ("" : : "x" (__x)); \ else /* long double */ \ /* "f": load x into (any) fpreg */ \ - __asm __volatile ("" : : "f" (x)); \ + __asm __volatile ("" : : "f" (__x)); \ } while (0) #endif -/* Default implementation forces store to a memory location */ +/* Default implementations force store to a memory location */ #ifndef math_opt_barrier #define math_opt_barrier(x) ({ __typeof(x) __x = (x); __asm ("" : "+m" (__x)); __x; }) -#define math_force_eval(x) __asm __volatile ("" : : "m" (x)) +#endif +#ifndef math_force_eval +#define math_force_eval(x) do { __typeof(x) __x = (x); __asm __volatile ("" : : "m" (__x)); } while (0) #endif |