When I tried to compile OpenWRT SVN trunk for my FON2100 with r16235, I got the following error messages:

 mips-openwrt-linux-uclibc-gcc -DHAVE_CONFIG_H -I. -I.. -I../src -I../src -I/opt/tmp/openwrt-svn-trunk-private/staging_dir/target-mips_uClibc-0.9.30.1/usr/include -I/opt/tmp/openwrt-svn-trunk-private/staging_dir/target-mips_uClibc-0.9.30.1/include -I/opt/tmp/openwrt-svn-trunk-private/staging_dir/target-mips_uClibc-0.9.30.1/usr/include -Os -pipe -mips32 -mtune=mips32 -funit-at-a-time -fhonour-copts -fpic -fvisibility=hidden -Wall -MT mpih-div.lo -MD -MP -MF .deps/mpih-div.Tpo -c mpih-div.c  -fPIC -DPIC -o .libs/mpih-div.o
mpih-div.c: In function '_gcry_mpih_mod_1':
mpih-div.c:98: error: impossible constraint in 'asm'
mpih-div.c:98: error: impossible constraint in 'asm'
mpih-div.c:104: error: impossible constraint in 'asm'
mpih-div.c:104: error: impossible constraint in 'asm'
mpih-div.c:134: error: impossible constraint in 'asm'
mpih-div.c:134: error: impossible constraint in 'asm'
make[5]: *** [mpih-div.lo] Error 1
make[5]: Leaving directory `/opt/tmp/openwrt-svn-trunk-private/build_dir/target-mips_uClibc-0.9.30.1/libgcrypt-1.4.4/mpi'
make[4]: *** [all-recursive] Error 1
make[4]: Leaving directory `/opt/tmp/openwrt-svn-trunk-private/build_dir/target-mips_uClibc-0.9.30.1/libgcrypt-1.4.4'
make[3]: *** [all] Error 2
make[3]: Leaving directory `/opt/tmp/openwrt-svn-trunk-private/build_dir/target-mips_uClibc-0.9.30.1/libgcrypt-1.4.4'
make[2]: *** [/opt/tmp/openwrt-svn-trunk-private/build_dir/target-mips_uClibc-0.9.30.1/libgcrypt-1.4.4/.built] Error 2
make[2]: Leaving directory `/opt/tmp/openwrt-svn-trunk-private/feeds/packages/libs/libgcrypt'
make[1]: *** [package/feeds/packages/libgcrypt/compile] Error 2
make[1]: Leaving directory `/opt/tmp/openwrt-svn-trunk-private'
make: *** [package/feeds/packages/libgcrypt/compile] Error 2

Does anyone know how to fix this issue?

Hi,

the reason for this error message is the gcc 4.4.0 change:
http://gcc.gnu.org/gcc-4.4/changes.html
...
The MIPS port no longer recognizes the h  asm constraint. It was necessary to remove this constraint in order to avoid generating unpredictable code sequences.

One of the main uses of the h constraint was to extract the high part of a multiplication on 64-bit targets. For example:

    asm ("dmultu\t%1,%2" : "=h" (result) : "r" (x), "r" (y));

You can now achieve the same effect using 128-bit types:

    typedef unsigned int uint128_t __attribute__((mode(TI)));
    result = ((uint128_t) x * y) >> 64;
...

You might trying this patch, (derivate of a patch applied to gmp library recently)
--- libgcrypt-1.4.4.orig/mpi/longlong.h    2008-08-19 17:20:03.000000000 +0200
+++ libgcrypt-1.4.4/mpi/longlong.h    2009-06-14 19:45:28.000000000 +0200
@@ -710,18 +710,35 @@ extern USItype __udiv_qrnnd ();
#endif /* __m88110__ */
#endif /* __m88000__ */

+/* Test for gcc >= maj.min, as per __GNUC_PREREQ in glibc */
+#if defined (__GNUC__) && defined (__GNUC_MINOR__)
+#define __GNUC_PREREQ(maj, min) \
+  ((__GNUC__ << 16) + __GNUC_MINOR__ >= ((maj) << 16) + (min))
+#else
+#define __GNUC_PREREQ(maj, min)  0
+#endif
+
/***************************************
  **************  MIPS  *****************
  ***************************************/
#if defined (__mips__) && W_TYPE_SIZE == 32
-#if __GNUC__ > 2 || __GNUC_MINOR__ >= 7
+#if __GNUC_PREREQ (4,4)
+#define umul_ppmm(w1, w0, u, v) \
+  do {                                                                 \
+    UDItype __ll = (UDItype)(u) * (v);                                 \
+    w1 = __ll >> 32;                                                   \
+    w0 = __ll;                                                         \
+  } while (0)
+#endif
+#if !defined (umul_ppmm) && __GNUC_PREREQ (2,7)
#define umul_ppmm(w1, w0, u, v) \
   __asm__ ("multu %2,%3"                                                \
       : "=l" ((USItype)(w0)),                                      \
         "=h" ((USItype)(w1))                                       \
       : "d" ((USItype)(u)),                                        \
         "d" ((USItype)(v)))
-#else
+#endif
+#if !defined (umul_ppmm)
#define umul_ppmm(w1, w0, u, v) \
   __asm__ ("multu %2,%3 \n" \
       "mflo %0 \n"     \
@@ -739,14 +756,24 @@ extern USItype __udiv_qrnnd ();
  **************  MIPS/64  **************
  ***************************************/
#if (defined (__mips) && __mips >= 3) && W_TYPE_SIZE == 64
-#if __GNUC__ > 2 || __GNUC_MINOR__ >= 7
+#if __GNUC_PREREQ (4,4)
+#define umul_ppmm(w1, w0, u, v) \
+  do {                                                                 \
+    typedef unsigned int __ll_UTItype __attribute__((mode(TI)));       \
+    __ll_UTItype __ll = (__ll_UTItype)(u) * (v);                       \
+    w1 = __ll >> 64;                                                   \
+    w0 = __ll;                                                         \
+  } while (0)
+#endif
+#if !defined (umul_ppmm) && __GNUC_PREREQ (2,7)
#define umul_ppmm(w1, w0, u, v) \
   __asm__ ("dmultu %2,%3"                                               \
       : "=l" ((UDItype)(w0)),                                      \
         "=h" ((UDItype)(w1))                                       \
       : "d" ((UDItype)(u)),                                        \
         "d" ((UDItype)(v)))
-#else
+#endif
+#if !defined (umul_ppmm)
#define umul_ppmm(w1, w0, u, v) \
   __asm__ ("dmultu %2,%3 \n"    \
       "mflo %0 \n"         \

Don't know if it works at runtime, at least you get an idea what needs to be done.