[OpenWrt-Devel] Build issue using ccache due to GCC patch / R_MIPS_26

[Karl Vogel]

There seems to be an issue using ccache in combination with the openwrt 
patched gcc.

Found the issue while investigating the following compilation issue:

---
openwrt/staging_dir/toolchain-mips_gcc-4.6-linaro_uClibc-0.9.33.2/lib/gcc/mips-openwrt-linux-uclibc/4.6.3/../../../../mips-openwrt-linux-uclibc/bin/ld: 
.libs/libxt_ACCOUNT_cl.o: relocation R_MIPS_26 against `memset' can not 
be used when making a shared object; recompile with -fPIC
libs/libxt_ACCOUNT_cl.o: could not read symbols: Bad value
collect2: ld returned 1 exit status
make[8]: *** [libxt_ACCOUNT_cl.la] Error 1
---


This only happened sometimes and only on certain machines. Closer look 
showed that libtool wasn't actually adding -fPIC. This was caused by 
the autoconf check for the PIC flag sometimes failing on my machine:

   checking for ccache_cc option to produce PIC... -fPIC -DPIC
   checking if ccache_cc PIC flag -fPIC -DPIC works... no

Doing a 'clean prepare compile' of the package sometimes fixed it.

The problem is that the autoconf test is doing a compilation with -fPIC 
then compares the stdout of the compiler to a 'normal' compilation. The 
results have to be equal to pass the test, in my setup, this wasn't 
always the case.

The reason was that the compilation output for that step sometimes gave 
a warning:

   warning: someone does not honour COPTS correctly, passed 2 times

This warning comes from the 910-mbsd_multi.patch patch to gcc, which 
verifies that -fhonour-copts is on the compilation line and that it is 
only present once. The patch also uses an environment variable called 
GCC_HONOUR_COPTS which indicates what has to happen if the option isn't 
present or is specified more than once. (possible values 0, 1, 2 and 
's' for silent)


The problem that was occuring on my setup was that -fhonour-copts had 
been specified more than once for an autoconf check and that this 
result was cached by ccache.

All subsequent compilations for the same autoconf conftest.c file, 
returned the result from the compiler cache as for ccache the same 
inputs where used for the compilation. The problem here is that ccache 
has no knowledge that the environment variable GCC_HONOUR_COPTS 
influences the stdout from gcc and since the stdout is of importance to 
autoconf, it was causing the PIC config test to fail due to a prior 
cached compilation result.

To verify my finding, I tried compilation without ccache, which worked 
and then tried patching ccache to also track GCC_HONOUR_COPTS, which 
also produced the correct result:

--- ccache-3.1.7/ccache.c       2012-01-08 15:40:55.000000000 +0100
+++ ccache-3.1.7-patched/ccache.c       2015-07-14 09:56:02.037675777 
+0200
@@ -965,6 +965,7 @@ calculate_object_hash(struct args *args,
                        "CPLUS_INCLUDE_PATH",
                        "OBJC_INCLUDE_PATH",
                        "OBJCPLUS_INCLUDE_PATH", /* clang */
+                        "GCC_HONOUR_COPTS",
                        NULL
                };
                for (p = envvars; *p != NULL ; ++p) {

With this patch, ccache will also add the GCC_HONOUR_COPTS setting to 
the hash of the compilation command and will consider 2 compilations 
with a different GCC_HONOUR_COPTS to actually be 2 different 
compilations.


However, this doesn't completely fix the problem as it appears that the 
tools/ccache/Makefile will only be compiled if ccache isn't already 
present on the host system, so probably the proper way to fix this is 
to just unconditionally compile ccache too if it is selected by the 
build system and then add the above ccache patch to the package.

Regards,
Karl
_______________________________________________
openwrt-devel mailing list
openwrt-devel at lists.openwrt.org
https://lists.openwrt.org/cgi-bin/mailman/listinfo/openwrt-devel