Additional performance improvements in sqlite3AtoF(). Also update the

header comments.

FossilOrigin-Name: 8c3a1b450d6d5778004b9fb0e92b4f3f90d3e06ff87276370d01a45b7d607d23

manifest

@@ -1,5 +1,5 @@
-C Further\ssimplification\sand\sperformance\simprovement
-D 2026-03-22T02:51:18.233
+C Additional\sperformance\simprovements\sin\ssqlite3AtoF().\s\sAlso\supdate\sthe\nheader\scomments.
+D 2026-03-22T17:50:48.147
 F .fossil-settings/binary-glob 61195414528fb3ea9693577e1980230d78a1f8b0a54c78cf1b9b24d0a409ed6a x
 F .fossil-settings/empty-dirs dbb81e8fc0401ac46a1491ab34a7f2c7c0452f2f06b54ebb845d024ca8283ef1
 F .fossil-settings/ignore-glob 35175cdfcf539b2318cb04a9901442804be81cd677d8b889fcc9149c21f239ea
@@ -798,7 +798,7 @@ F src/trigger.c 4bf3bfb3851d165e4404a9f9e69357345f3f7103378c07e07139fdd8aeb7bd20
 F src/update.c 3e5e7ff66fa19ebe4d1b113d480639a24cc1175adbefabbd1a948a07f28e37cf
 F src/upsert.c 215328c3f91623c520ec8672c44323553f12caeb4f01b1090ebdca99fdf7b4f1
 F src/utf.c 7267c3fb9e2467020507601af3354c2446c61f444387e094c779dccd5ca62165
-F src/util.c 44aeebb0daa0ea3109daadff33399014378e05ced9220f6151fa03c2fa4380cc
+F src/util.c b7ec2de13f540ca184173f44845888f5b727f92b9d7255ca7309bec7ed0bf184
 F src/vacuum.c d3d35d8ae893d419ade5fa196d761a83bddcbb62137a1a157ae751ef38b26e82
 F src/vdbe.c dbafc6f5762f9ae90d9dc9ed065a31743a8ceafbdbfb1a0a3a228921fb3cc36a
 F src/vdbe.h 70e862ac8a11b590f8c1eaac17a0078429d42bc4ea3f757a9af0f451dd966a71
@@ -2195,8 +2195,8 @@ F tool/warnings-clang.sh bbf6a1e685e534c92ec2bfba5b1745f34fb6f0bc2a362850723a9ee
 F tool/warnings.sh a554d13f6e5cf3760f041b87939e3d616ec6961859c3245e8ef701d1eafc2ca2
 F tool/win/sqlite.vsix deb315d026cc8400325c5863eef847784a219a2f
 F tool/winmain.c 00c8fb88e365c9017db14c73d3c78af62194d9644feaf60e220ab0f411f3604c
-P aea648f3668219a2539160eb68c77ddd06aa8eac235c1326fe7b1547288b11ee
-R 87703e2fc96cf20793c6348cd259dd70
+P 6c3dc2430adef061b458b6d9159628bb8debfd4d63876d5d5e7493afb799d97f
+R 0c1564cd8bd4be660fcfe28f92df6384
 U drh
-Z 772c7a9e66852f7312caf69be9a36c21
+Z c76030a82efd805d9e5d4b632bb41994
 # Remove this line to create a well-formed Fossil manifest.

manifest.uuid

@@ -1 +1 @@
-6c3dc2430adef061b458b6d9159628bb8debfd4d63876d5d5e7493afb799d97f
+8c3a1b450d6d5778004b9fb0e92b4f3f90d3e06ff87276370d01a45b7d607d23

src/util.c

@@ -764,6 +764,9 @@ static void sqlite3Fp2Convert10(u64 m, int e, int n, u64 *pD, int *pP){
 
 /*
 ** Return an IEEE754 floating point value that approximates d*pow(10,p).
+**
+** The (current) algorithm is adapted from the work of Ross Cox at
+** https://github.com/rsc/fpfmt
 */
 static double sqlite3Fp10Convert2(u64 d, int p){
   int b, lp, e, adj, s;
@@ -815,14 +818,17 @@ static double sqlite3Fp10Convert2(u64 d, int p){
 **
 ** z[] must be UTF-8 and zero-terminated.
 **
-** Return TRUE if the result is a valid real number (or integer) and FALSE
-** if the string is empty or contains extraneous text.  More specifically
-** return
+** Return positive if the result is a valid real number (or integer) and
+** zero or negative if the string is empty or contains extraneous text.
+** More specifically:
+**
 **      1          =>  The input string is a pure integer
 **      2 or more  =>  The input has a decimal point or eNNN clause
-**      0 or less  =>  The input string is not a valid number
-**     -1          =>  Not a valid number, but has a valid prefix which
-**                     includes a decimal point and/or an eNNN clause
+**      0 or less  =>  The input string is not well-formed
+**     -1          =>  The input is not well-formed, but it does begin
+**                     with a well-formed floating-point literal (with
+**                     a "." or a "eNNN" suffix or both) followed by
+**                     other extraneous text.
 **
 ** Valid numbers are in one of these formats:
 **
@@ -833,29 +839,38 @@ static double sqlite3Fp10Convert2(u64 d, int p){
 ** Leading and trailing whitespace is ignored for the purpose of determining
 ** validity.
 **
-** If some prefix of the input string is a valid number, this routine
-** returns FALSE but it still converts the prefix and writes the result
-** into *pResult.
+** Algorithm sketch:  Compute an unsigned 64-bit integer s and a base-10
+** exponent d such that the value encoding by the input is s*pow(10,d).
+** Then invoke sqlite3Fp10Convert2() to calculated the closest possible
+** IEEE754 double.  The sign is added back afterwards, if the input string
+** starts with a "-".  The use of an unsigned 64-bit s mantissa means that
+** only about the first 19 significant digits of the input can contribute
+** to the result.  This can result in suboptimal rounding decisions when
+** correct rounding requires more than 19 input digits.  For example,
+** this routine renders "3500000000000000.2500001" as
+** 3500000000000000.0 instead of 3500000000000000.5 because the decision
+** to round up instead of using banker's rounding to round down is determined
+** by the 23rd significant digit, which this routine ignores. It is not
+** possible to do better without some kind of BigNum.
 */
-#if defined(_MSC_VER)
-#pragma warning(disable : 4756)
-#endif
-int sqlite3AtoF(const char *z, double *pResult){
+int sqlite3AtoF(const char *zIn, double *pResult){
 #ifndef SQLITE_OMIT_FLOATING_POINT
+  const unsigned char *z = (const unsigned char*)zIn;
   int neg = 0;       /* True for a negative value */
   u64 s = 0;         /* mantissa */
   int d = 0;         /* Value is s * pow(10,d) */
   int seenDigit = 0; /* true if any digits seen */
   int seenFP = 0;    /* True if we've seen a "." or a "e" */
+  unsigned v;        /* Value of a single digit */
 
   start_of_text:
-  if( sqlite3Isdigit(z[0]) ){
+  if( (v = (unsigned)z[0] - '0')<10 ){
     parse_integer_part:
     seenDigit = 1;
-    s = z[0] - '0';
+    s = v;
     z++;
-    while( sqlite3Isdigit(z[0]) ){
-      s = s*10 + (z[0] - '0');
+    while( (v = (unsigned)z[0] - '0')<10 ){
+      s = s*10 + v;
       z++;
       if( s>=(LARGEST_INT64-9)/10 ){
         while( sqlite3Isdigit(z[0]) ){ z++; d++; }
@@ -865,10 +880,10 @@ int sqlite3AtoF(const char *z, double *pResult){
   }else if( z[0]=='-' ){
     neg = 1;
     z++;
-    if( sqlite3Isdigit(z[0]) ) goto parse_integer_part;
+    if( (v = (unsigned)z[0] - '0')<10 ) goto parse_integer_part;
   }else if( z[0]=='+' ){
     z++;
-    if( sqlite3Isdigit(z[0]) ) goto parse_integer_part;
+    if( (v = (unsigned)z[0] - '0')<10 ) goto parse_integer_part;
   }else if( sqlite3Isspace(z[0]) ){
     do{ z++; }while( sqlite3Isspace(z[0]) );
     goto start_of_text;
@@ -913,12 +928,12 @@ int sqlite3AtoF(const char *z, double *pResult){
       }
     }
     /* copy digits to exponent */
-    if( sqlite3Isdigit(*z) ){
-      int exp = *z - '0';
+    if( (v = (unsigned)z[0] - '0')<10 ){
+      int exp = v;
       z++;
       seenFP = 1;
-      while( sqlite3Isdigit(*z) ){
-        exp = exp<10000 ? (exp*10 + (*z - '0')) : 10000;
+      while( (v = (unsigned)z[0] - '0')<10 ){
+        exp = exp<10000 ? (exp*10 + v) : 10000;
         z++;
       }
       d += esign*exp;