[Patch, avr] Emit diagnostics if -f{pic,PIC,pie,PIE} or -shared is passed

[Senthil Kumar Selvaraj]

The AVR backend does not generate position independent code, yet it
happily accepts -fpic, -fPIC, -fpie and -fPIE. The generated code
doesn't change at all. Also, it accepts the -shared option to generate a
shared library, without really doing anything with it.

This causes one of the regression tests 
(gcc.dg/lto/pr54709 c_lto_pr54709_0.o-c_lto_pr54709_1.o link) to fail with 
an 'undefined reference to main' error, when the test is trying to build 
a shared object.

The attached patch generates a warning if one of the -f{pic,PIC,pie,PIE} 
options is provided, and an error if -shared is provided (
config/mep/mep.c and config/s390/tpf.h already do something very similar).

Regression tested with no new failures.Tests which exercise PIC now report as 
unsupported.

If ok, could someone commit please? I don't have commit access.

Regards
Senthil

gcc/ChangeLog
2013-11-04  Senthil Kumar Selvaraj  <senthil_kumar.selva...@atmel.com>

        * config/avr/avr.c (avr_option_override): Warn if asked to generate
        position independent code.
        * config/avr/avr.h: Modify LINK_SPEC to reject -shared.


diff --git gcc/config/avr/avr.c gcc/config/avr/avr.c
index e7e1c2f..cf4b8ca 100644
--- gcc/config/avr/avr.c
+++ gcc/config/avr/avr.c
@@ -310,6 +310,15 @@ avr_option_override (void)
       flag_omit_frame_pointer = 0;
     }
 
+  if (flag_pic == 1)
+    warning (OPT_fpic, "-fpic is not supported");
+  if (flag_pic == 2)
+    warning (OPT_fPIC, "-fPIC is not supported");
+  if (flag_pie == 1)
+    warning (OPT_fpie, "-fpie is not supported");
+  if (flag_pie == 2)
+    warning (OPT_fPIE, "-fPIE is not supported");
+
   avr_current_device = &avr_mcu_types[avr_mcu_index];
   avr_current_arch = &avr_arch_types[avr_current_device->arch];
 
diff --git gcc/config/avr/avr.h gcc/config/avr/avr.h
index f223a61..1eff5be 100644
--- gcc/config/avr/avr.h
+++ gcc/config/avr/avr.h
@@ -522,7 +522,8 @@ extern const char *avr_device_to_sp8 (int argc, const char 
**argv);
                                mmcu=at90can64*|\
                                mmcu=at90usb64*:--pmem-wrap-around=64k}}}\
 %:device_to_ld(%{mmcu=*:%*})\
-%:device_to_data_start(%{mmcu=*:%*})"
+%:device_to_data_start(%{mmcu=*:%*})\
+%{shared:%eshared is not supported}"
 
 #define LIB_SPEC \
   
"%{!mmcu=at90s1*:%{!mmcu=attiny11:%{!mmcu=attiny12:%{!mmcu=attiny15:%{!mmcu=attiny28:
 -lc }}}}}"