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-rw-r--r--libm/math_private.h44
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