avr: Fix wrong array bounds warning on SFR access

The warning was raised on accessing SFRs at addresses below the default
page size, as gcc considers accessing addresses in the first page of
memory as suspicious. This doesn't apply to an embedded target like the
avr, where both flash and RAM have zero as a valid address. Zero is also
a valid address in named address spaces (__memx, flash<n> etc..).

This commit implements TARGET_ADDR_SPACE_ZERO_ADDRESS_VALID for the avr
target and reports to gcc that zero is a valid address on all
address spaces. It also disables flag_delete_null_pointer_checks
based on the target hook, and modifies target-supports.exp to add avr
to the list of targets that always keep null pointer checks. This fixes
a bunch of DejaGNU failures that occur otherwise.

	PR target/105523

gcc/ChangeLog:

	* common/config/avr/avr-common.cc: Remove setting
	of OPT_fdelete_null_pointer_checks.
	* config/avr/avr.cc (avr_option_override): Clear
	flag_delete_null_pointer_checks if zero_address_valid.
	(avr_addr_space_zero_address_valid): New function.
	(TARGET_ADDR_SPACE_ZERO_ADDRESS_VALID): Provide target
	hook.

gcc/testsuite/ChangeLog:

	* lib/target-supports.exp
	(check_effective_target_keeps_null_pointer_checks): Add
	avr.
	* gcc.target/avr/pr105523.c: New test.

gcc/common/config/avr/avr-common.cc

@@ -29,12 +29,6 @@
 /* Implement TARGET_OPTION_OPTIMIZATION_TABLE.  */
 static const struct default_options avr_option_optimization_table[] =
   {
-    // With -fdelete-null-pointer-checks option, the compiler assumes
-    // that dereferencing of a null pointer would halt the program.
-    // For AVR this assumption is not true and a program can safely
-    // dereference null pointers.  Changes made by this option may not
-    // work properly for AVR.  So disable this option.
-    { OPT_LEVELS_ALL, OPT_fdelete_null_pointer_checks, NULL, 0 },
     // The only effect of -fcaller-saves might be that it triggers
     // a frame without need when it tries to be smart around calls.
     { OPT_LEVELS_ALL, OPT_fcaller_saves, NULL, 0 },

gcc/config/avr/avr.cc

@@ -1095,6 +1095,10 @@ avr_option_override (void)
       flag_omit_frame_pointer = 0;
     }
 
+  /* Disable flag_delete_null_pointer_checks if zero is a valid address. */
+  if (targetm.addr_space.zero_address_valid (ADDR_SPACE_GENERIC))
+    flag_delete_null_pointer_checks = 0;
+
   if (flag_pic == 1)
     warning (OPT_fpic, "%<-fpic%> is not supported");
   if (flag_pic == 2)
@@ -10501,6 +10505,16 @@ avr_addr_space_diagnose_usage (addr_space_t as, location_t loc)
   (void) avr_addr_space_supported_p (as, loc);
 }
 
+/* Implement `TARGET_ADDR_SPACE_ZERO_ADDRESS_VALID. Zero is a valid
+   address in all address spaces. Even in ADDR_SPACE_FLASH1 etc..,
+   a zero address is valid and means 0x<RAMPZ val>0000, where RAMPZ is
+   set to the appropriate segment value. */
+
+static bool
+avr_addr_space_zero_address_valid (addr_space_t)
+{
+  return true;
+}
 
 /* Look if DECL shall be placed in program memory space by
    means of attribute `progmem' or some address-space qualifier.
@@ -15255,6 +15269,9 @@ avr_float_lib_compare_returns_bool (machine_mode mode, enum rtx_code)
 #undef  TARGET_ADDR_SPACE_DIAGNOSE_USAGE
 #define TARGET_ADDR_SPACE_DIAGNOSE_USAGE avr_addr_space_diagnose_usage
 
+#undef  TARGET_ADDR_SPACE_ZERO_ADDRESS_VALID
+#define TARGET_ADDR_SPACE_ZERO_ADDRESS_VALID avr_addr_space_zero_address_valid
+
 #undef  TARGET_MODE_DEPENDENT_ADDRESS_P
 #define TARGET_MODE_DEPENDENT_ADDRESS_P avr_mode_dependent_address_p
 

gcc/testsuite/gcc.target/avr/pr105523.c

@@ -0,0 +1,14 @@
+/* { dg-do compile } */
+/* { dg-options "-Os -Wall" } */
+
+/* Verify no "array subscript 0 is outside array bounds of" is generated
+   for accessing memory addresses in the 0-4096 range. */
+
+typedef __UINT8_TYPE__ uint8_t;
+
+#define SREG (*(volatile uint8_t*) (0x3F + __AVR_SFR_OFFSET__ ))
+
+void bar (void)
+{
+    SREG = 0;
+}

gcc/testsuite/lib/target-supports.exp

@@ -686,7 +686,8 @@ proc check_effective_target_keeps_null_pointer_checks { } {
     if [target_info exists keeps_null_pointer_checks] {
       return 1
     }
-    if { [istarget msp430-*-*] } {
+    if { [istarget msp430-*-*]
+         || [istarget avr-*-*] } {
 	return 1;   
     }
     return 0