Very small numbers in hexadecimal notation parsed as zero

Hi all,

I have noticed incorrect parsing of very small hexadecimal numbers
like "0x1.00000000d0000p-987". Such a hexadecimal representation can
can be produced by sprintf() using the %a flag. The return value is
incorrectly reported as 0 when coercing these numbers to double using
as.double()/as.numeric(), as illustrated in the three examples below:

as.double("0x1.00000000d0000p-987")    # should be 7.645296e-298
as.double("0x1.0000000000000p-1022")  # should be 2.225074e-308
as.double("0x1.f89fc1a6f6613p-974")      # should be 1.23456e-293

The culprit seems to be the src/main/util.c:R_strtod function and in
some cases, removing the zeroes directly before the 'p' leads to
correct parsing:

as.double("0x1.00000000dp-987") # 7.645296e-298, as expected
as.double("0x1.p-1022")               # 2.225074e-308, as expected

I wrote a small program (in a file called "strtod.c") to compare the R
stdtod implementation to a C implementation. The C implementation
never reported 0 in the examples given above:

#include <stdlib.h>
#include <stdio.h>
int main(void)
{
   char *string, *stopstring;
   double x;

   string = "0x1.00000000d0000p-987";
   x = strtod(string, &stopstring);
   printf("string = \"%s\"\n", string);
   printf("strtod = %.17g\n\n", x);

   string = "0x1.00000000dp-987";
   x = strtod(string, &stopstring);
   printf("string = \"%s\"\n", string);
   printf("strtod = %.17g\n\n", x);
}

$ gcc -o strtod.exe strtod.c
$ ./strtod.exe
string = "0x1.00000000d0000p-987"
strtod = 7.6452955642246671e-298

string = "0x1.00000000dp-987"
strtod = 7.6452955642246671e-298

string = "0x1.0000000000000p-1022"
strtod = 2.2250738585072014e-308

string = "0x1.p-1022"
strtod = 2.2250738585072014e-308

string = "0x1.f89fc1a6f6613p-974"
strtod = 1.23456e-293


My sessionInfo() returns:
R version 3.3.2 (2016-10-31)
Platform: x86_64-w64-mingw32/x64 (64-bit)
Running under: Windows 7 x64 (build 7601) Service Pack 1

locale:
[1] LC_COLLATE=German_Switzerland.1252
LC_CTYPE=German_Switzerland.1252
LC_MONETARY=German_Switzerland.1252 LC_NUMERIC=C
 LC_TIME=German_Switzerland.1252

attached base packages:
[1] stats     graphics  grDevices utils     datasets  methods   base


Regards,
Florent