Re: [PATCH] avr: Set param_min_pagesize to 0 [PR105523]

[<SenthilKumar.Selvaraj@microchip.com>]

On Mon, 2023-05-22 at 14:05 +0200, Richard Biener wrote:
> EXTERNAL EMAIL: Do not click links or open attachments unless you know the content is safe
> 
> On Fri, May 19, 2023 at 7:58 AM <SenthilKumar.Selvaraj@microchip.com> wrote:
> > On 26/04/23, 5:51 PM, "Richard Biener" <richard.guenther@gmail.com <mailto:richard.guenther@gmail.com>> wrote:
> > > On Wed, Apr 26, 2023 at 12:56 PM <SenthilKumar.Selvaraj@microchip.com <mailto:SenthilKumar.Selvaraj@microchip.com>> wrote:
> > > > On Wed, Apr 26, 2023 at 3:15 PM Richard Biener via Gcc-patches <gcc-patches@gcc.gnu.org <mailto:gcc-patches@gcc.gnu.org>> wrote:
> > > > > On Wed, Apr 26, 2023 at 11:42 AM Richard Biener
> > > > > <richard.guenther@gmail.com <mailto:richard.guenther@gmail.com>> wrote:
> > > > > > On Wed, Apr 26, 2023 at 11:01 AM SenthilKumar.Selvaraj--- via
> > > > > > Gcc-patches <gcc-patches@gcc.gnu.org <mailto:gcc-patches@gcc.gnu.org>> wrote:
> > > > > > > Hi,
> > > > > > > 
> > > > > > > This patch fixes PR 105523 by setting param_min_pagesize to 0 for the
> > > > > > > avr target. For this target, zero and offsets from zero are perfectly
> > > > > > > valid addresses, and the default value of param_min_pagesize ends up
> > > > > > > triggering warnings on valid memory accesses.
> > > > > > 
> > > > > > I think the proper configuration is to have
> > > > > > DEFAULT_ADDR_SPACE_ZERO_ADDRESS_VALID
> > > > > 
> > > > > Err, TARGET_ADDR_SPACE_ZERO_ADDRESS_VALID
> > > > 
> > > > That worked. Ok for trunk and backporting to 13 and 12 branches
> > > > (pending regression testing)?
> > > 
> > > OK, but please let Denis time to comment.
> > 
> > Didn't hear from Denis. When running regression tests with this patch,
> > I found that some tests with -fdelete-null-pointer-checks were
> > failing. Commit 19416210b37db0584cd0b3f3b3961324b8973d25 made
> > -fdelete-null-pointer-checks false by default, while still allowing it
> > to be overridden from the command line (it was previously
> > unconditionally false).
> > 
> > To keep the same behavior, I modified the hook to report zero
> > addresses as valid only if -fdelete-null-pointer-checks is not set.
> > With this change, all regression tests pass.
> > 
> > Ok for trunk and backporting to 13 and 12 branches?
> 
> I think that's bit backwards - this hook conveys more precise information
> (it's address-space specific) and it is also more specific.  Instead I'd
> suggest to set the flag to zero in the target like nios2 or msp430 do.
> In fact we should probably initialize it using this hook (and using the
> default address space).

Does the below patch work? The hook impl reports that zero address is
valid, and flag_delete_null_pointer_checks is set to zero if the
hook says zero is a valid address.

As flag_delete_null_pointer_checks is now always disabled for avr, I
removed the resetting code in avr-common.cc that disables it for
OPT_LEVELS_ALL by default, and added avr as a target that always keeps
null pointer checks in testsuite/lib/target-supports.exp.

I also removed ATTRIBUTE_UNUSED and the parameter name in the target
hook to address https://gcc.gnu.org/pipermail/gcc-patches/2023-May/619014.html. 

PR 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 --git gcc/common/config/avr/avr-common.cc gcc/common/config/avr/avr-common.cc
index 2ad0244..2f874c5 100644
--- gcc/common/config/avr/avr-common.cc
+++ 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 gcc/config/avr/avr.cc gcc/config/avr/avr.cc
index a90cade..b987837 100644
--- gcc/config/avr/avr.cc
+++ gcc/config/avr/avr.cc
@@ -756,6 +756,10 @@ 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)
@@ -9800,6 +9804,16 @@ 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.
@@ -14760,6 +14774,9 @@ 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 gcc/testsuite/gcc.target/avr/pr105523.c gcc/testsuite/gcc.target/avr/pr105523.c
new file mode 100644
index 0000000..fbbf7bf
--- /dev/null
+++ 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 gcc/testsuite/lib/target-supports.exp gcc/testsuite/lib/target-supports.exp
index 263ef35..56fa73d 100644
--- gcc/testsuite/lib/target-supports.exp
+++ 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