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, 3 insertions, 2 deletions

diff --git a/src/script_lua.c b/src/script_lua.c
index 67209615b..70c7185f9 100644
--- a/src/script_lua.c
+++ b/src/script_lua.c
@@ -28,6 +28,7 @@
  */
 
 #include "script_lua.h"
+#include "fpconv_dtoa.h"
 
 #include "server.h"
 #include "sha1.h"
@@ -804,8 +805,8 @@ static robj **luaArgsToRedisArgv(lua_State *lua, int *argc) {
             /* We can't use lua_tolstring() for number -> string conversion
              * since Lua uses a format specifier that loses precision. */
             lua_Number num = lua_tonumber(lua,j+1);
-
-            obj_len = snprintf(dbuf,sizeof(dbuf),"%.17g",(double)num);
+            obj_len = fpconv_dtoa((double)num, dbuf);
+            dbuf[obj_len] = '\0';
             obj_s = dbuf;
         } else {
             obj_s = (char*)lua_tolstring(lua,j+1,&obj_len);