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

Author: filipecosta90

.gitignore

@@ -29,6 +29,7 @@ deps/lua/src/lua
 deps/lua/src/luac
 deps/lua/src/liblua.a
 deps/hdr_histogram/libhdrhistogram.a
+deps/fpconv/libfpconv.a
 tests/tls/*
 .make-*
 .prerequisites

deps/Makefile

@@ -40,6 +40,7 @@ distclean:
 	-(cd lua && $(MAKE) clean) > /dev/null || true
 	-(cd jemalloc && [ -f Makefile ] && $(MAKE) distclean) > /dev/null || true
 	-(cd hdr_histogram && $(MAKE) clean) > /dev/null || true
+	-(cd fpconv && $(MAKE) clean) > /dev/null || true
 	-(rm -f .make-*)
 
 .PHONY: distclean
@@ -66,6 +67,12 @@ hdr_histogram: .make-prerequisites
 
 .PHONY: hdr_histogram
 
+fpconv: .make-prerequisites
+	@printf '%b %b\n' $(MAKECOLOR)MAKE$(ENDCOLOR) $(BINCOLOR)$@$(ENDCOLOR)
+	cd fpconv && $(MAKE)
+
+.PHONY: fpconv
+
 ifeq ($(uname_S),SunOS)
 	# Make isinf() available
 	LUA_CFLAGS= -D__C99FEATURES__=1

deps/fpconv/LICENSE.txt

@@ -0,0 +1,23 @@
+Boost Software License - Version 1.0 - August 17th, 2003
+
+Permission is hereby granted, free of charge, to any person or organization
+obtaining a copy of the software and accompanying documentation covered by
+this license (the "Software") to use, reproduce, display, distribute,
+execute, and transmit the Software, and to prepare derivative works of the
+Software, and to permit third-parties to whom the Software is furnished to
+do so, all subject to the following:
+
+The copyright notices in the Software and this entire statement, including
+the above license grant, this restriction and the following disclaimer,
+must be included in all copies of the Software, in whole or in part, and
+all derivative works of the Software, unless such copies or derivative
+works are solely in the form of machine-executable object code generated by
+a source language processor.
+
+THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+FITNESS FOR A PARTICULAR PURPOSE, TITLE AND NON-INFRINGEMENT. IN NO EVENT
+SHALL THE COPYRIGHT HOLDERS OR ANYONE DISTRIBUTING THE SOFTWARE BE LIABLE
+FOR ANY DAMAGES OR OTHER LIABILITY, WHETHER IN CONTRACT, TORT OR OTHERWISE,
+ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER
+DEALINGS IN THE SOFTWARE.

deps/fpconv/Makefile

@@ -0,0 +1,27 @@
+STD=
+WARN= -Wall
+OPT= -Os
+
+R_CFLAGS= $(STD) $(WARN) $(OPT) $(DEBUG) $(CFLAGS)
+R_LDFLAGS= $(LDFLAGS)
+DEBUG= -g
+
+R_CC=$(CC) $(R_CFLAGS)
+R_LD=$(CC) $(R_LDFLAGS)
+
+AR= ar
+ARFLAGS= rcs
+
+libfpconv.a: fpconv_dtoa.o
+	$(AR) $(ARFLAGS) $@ $+
+
+fpconv_dtoa.o: fpconv_dtoa.h fpconv_dtoa.c
+
+.c.o:
+	$(R_CC) -c  $< 
+
+clean:
+	rm -f *.o
+	rm -f *.a
+
+

deps/fpconv/README.md

@@ -0,0 +1,9 @@
+libfpconv
+
+----------------------------------------------
+
+Fast and accurate double to string conversion based on Florian Loitsch's Grisu-algorithm[1].
+
+This port contains a subset of the 'C' version of Fast and accurate double to string conversion based on Florian Loitsch's Grisu-algorithm available at [github.com/night-shift/fpconv](https://github.com/night-shift/fpconv)).
+
+[1] https://www.cs.tufts.edu/~nr/cs257/archive/florian-loitsch/printf.pdf