[gcc r14-1933] avr: Fix wrong array bounds warning on SFR access

[gcc r14-1933] avr: Fix wrong array bounds warning on SFR access

[Senthil Kumar Selvaraj]

https://gcc.gnu.org/g:58e1bc2b1c8420773b16452d47932a6ca0d003fb

commit r14-1933-g58e1bc2b1c8420773b16452d47932a6ca0d003fb
Author: Senthil Kumar Selvaraj <saaadhu@gcc.gnu.org>
Date:   Mon Jun 19 12:23:25 2023 +0530

    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.

Diff:
---
 gcc/common/config/avr/avr-common.cc     |  6 ------
 gcc/config/avr/avr.cc                   | 17 +++++++++++++++++
 gcc/testsuite/gcc.target/avr/pr105523.c | 14 ++++++++++++++
 gcc/testsuite/lib/target-supports.exp   |  3 ++-
 4 files changed, 33 insertions(+), 7 deletions(-)

diff --git a/gcc/common/config/avr/avr-common.cc b/gcc/common/config/avr/avr-common.cc
index 2ad02448e29..2f874c5b81c 100644
--- a/gcc/common/config/avr/avr-common.cc
+++ b/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 },
diff --git a/gcc/config/avr/avr.cc b/gcc/config/avr/avr.cc
index ef6872a3f55..0447641a8e9 100644
--- a/gcc/config/avr/avr.cc
+++ b/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
 
diff --git a/gcc/testsuite/gcc.target/avr/pr105523.c b/gcc/testsuite/gcc.target/avr/pr105523.c
new file mode 100644
index 00000000000..fbbf7bf4422
--- /dev/null
+++ b/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;
+}
diff --git a/gcc/testsuite/lib/target-supports.exp b/gcc/testsuite/lib/target-supports.exp
index ab2616a9a34..77822499bcf 100644
--- a/gcc/testsuite/lib/target-supports.exp
+++ b/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