msvcrt: Perform a base-10 logarithm using integer math.

dlls/msvcrt/bnum.h

@@ -22,7 +22,7 @@
 #define EXP_BITS 11
 #define MANT_BITS 53
 
-static const int p10s[] = { 1, 10, 100, 1000, 10000, 100000, 1000000, 10000000, 100000000 };
+static const int p10s[] = { 1, 10, 100, 1000, 10000, 100000, 1000000, 10000000, 100000000, 1000000000 };
 
 #define LIMB_DIGITS 9           /* each DWORD stores up to 9 digits */
 #define LIMB_MAX 1000000000     /* 10^9 */

dlls/msvcrt/printf.h

@@ -135,6 +135,20 @@ static inline int mbstowcs_len(wchar_t *wcstr, const char *mbstr,
     }
     return wlen;
 }
+
+static inline unsigned int log2i(unsigned int x)
+{
+    ULONG result;
+    _BitScanReverse(&result, x);
+    return result;
+}
+
+static inline unsigned int log10i(unsigned int x)
+{
+    unsigned int t = ((log2i(x) + 1) * 1233) / 4096;
+    return t - (x < p10s[t]);
+}
+
 #endif
 
 static inline int FUNC_NAME(pf_output_wstr)(FUNC_NAME(puts_clbk) pf_puts, void *puts_ctx,
@@ -635,7 +649,7 @@ static inline int FUNC_NAME(pf_output_fp)(FUNC_NAME(puts_clbk) pf_puts, void *pu
     if(!b->data[bnum_idx(b, b->e-1)])
         first_limb_len = 1;
     else
-        first_limb_len = floor(log10(b->data[bnum_idx(b, b->e - 1)])) + 1;
+        first_limb_len = log10i(b->data[bnum_idx(b, b->e - 1)]) + 1;
     radix_pos = first_limb_len + LIMB_DIGITS + e10;
 
     round_pos = flags->Precision;
@@ -700,7 +714,7 @@ static inline int FUNC_NAME(pf_output_fp)(FUNC_NAME(puts_clbk) pf_puts, void *pu
                 if(!b->data[bnum_idx(b, b->e-1)])
                     i = 1;
                 else
-                    i = floor(log10(b->data[bnum_idx(b, b->e-1)])) + 1;
+                    i = log10i(b->data[bnum_idx(b, b->e-1)]) + 1;
                 if(i != first_limb_len) {
                     first_limb_len = i;
                     radix_pos++;