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, 9 insertions, 3 deletions

diff --git a/src/networking.c b/src/networking.c
index 3cdbe16ce..180782584 100644
--- a/src/networking.c
+++ b/src/networking.c
@@ -31,6 +31,7 @@
 #include "atomicvar.h"
 #include "cluster.h"
 #include "script.h"
+#include "fpconv_dtoa.h"
 #include <sys/socket.h>
 #include <sys/uio.h>
 #include <math.h>
@@ -855,7 +856,7 @@ void addReplyDouble(client *c, double d) {
              * but still avoid an extra memcpy of the whole number, we reserve space
              * for maximum header `$0000\r\n`, print double, add the resp header in
              * front of it, and then send the buffer with the right `start` offset. */
-            int dlen = snprintf(dbuf+7,sizeof(dbuf) - 7,"%.17g",d);
+            dlen = fpconv_dtoa(d, dbuf+7);
             int digits = digits10(dlen);
             int start = 4 - digits;
             dbuf[start] = '$';
@@ -870,10 +871,15 @@ void addReplyDouble(client *c, double d) {
             dbuf[6] = '\n';
             dbuf[dlen+7] = '\r';
             dbuf[dlen+8] = '\n';
+            dbuf[dlen+9] = '\0';
             addReplyProto(c,dbuf+start,dlen+9-start);
         } else {
-            dlen = snprintf(dbuf,sizeof(dbuf),",%.17g\r\n",d);
-            addReplyProto(c,dbuf,dlen);
+            dbuf[0] = ',';
+            dlen = fpconv_dtoa(d, dbuf+1);
+            dbuf[dlen+1] = '\r';
+            dbuf[dlen+2] = '\n';
+            dbuf[dlen+3] = '\0';
+            addReplyProto(c,dbuf,dlen+3);
         }
     }
 }