optimizing d2string() and addReplyDouble() with grisu2: double to string conversion based on Florian Loitsch's Grisu-algorithm (#10587)

All commands / use cases that heavily rely on double to a string representation conversion,
(e.g. meaning take a double-precision floating-point number like 1.5 and return a string like "1.5" ),
could benefit from a performance boost by swapping snprintf(buf,len,"%.17g",value) by the
equivalent [fpconv_dtoa](https://github.com/night-shift/fpconv) or any other algorithm that ensures
100% coverage of conversion.

This is a well-studied topic and Projects like MongoDB. RedPanda, PyTorch leverage libraries
( fmtlib ) that use the optimized double to string conversion underneath.


The positive impact can be substantial. This PR uses the grisu2 approach ( grisu explained on
https://www.cs.tufts.edu/~nr/cs257/archive/florian-loitsch/printf.pdf section 5 ). 

test suite changes:
Despite being compatible, in some cases it produces a different result from printf, and some tests
had to be adjusted.
one case is that `%.17g` (which means %e or %f which ever is shorter), chose to use `5000000000`
instead of 5e+9, which sounds like a bug?
In other cases, we changed TCL to compare numbers instead of strings to ignore minor rounding
issues (`expr 0.8 == 0.79999999999999999`) 

1 files changed, 5 insertions, 2 deletions

diff --git a/src/rdb.c b/src/rdb.c
index 77499c39e..24628a89b 100644
--- a/src/rdb.c
+++ b/src/rdb.c
@@ -31,6 +31,7 @@
 #include "lzf.h"    /* LZF compression library */
 #include "zipmap.h"
 #include "endianconv.h"
+#include "fpconv_dtoa.h"
 #include "stream.h"
 #include "functions.h"
 
@@ -590,8 +591,10 @@ int rdbSaveDoubleValue(rio *rdb, double val) {
         /* Integer printing function is much faster, check if we can safely use it. */
         if (double2ll(val, &lvalue))
             ll2string((char*)buf+1,sizeof(buf)-1,lvalue);
-        else
-            snprintf((char*)buf+1,sizeof(buf)-1,"%.17g",val);
+        else {
+            const int dlen = fpconv_dtoa(val, (char*)buf+1);
+            buf[dlen+1] = '\0';
+        }
         buf[0] = strlen((char*)buf+1);
         len = buf[0]+1;
     }