[PATCH] PR target/82624 msp430-elf target must allow for NULL pointer dereferences

[Orlando Arias <oarias at knights dot ucf dot edu>]

Greetings,

As requested by Andrew Pinski, I should send the patch to this mailing
list. The patch was also included in the bug report alongside an
explanation as to why it is necessary [1].

In brief, the memory map of the MSP430 reserves the area from address
0x0000 to address 0x000f for special function registers. Software
attempting to access address 0x0000 as an implicitly declared pointer
will cause the compiler to silently generate bad code (gcc7) or to throw
an error message (gcc8 behavior, I think) when compiled with -O2 or
higher due to the -fdelete-null-pointer-checks optimization.

The attached patch disables the optimization for the msp430-elf target.
Please let me know if any changes are needed (also, keep me CC'd if
necessary, I am not a member of this mailing list). Thank you.

Cheers,
Orlando.

[1] https://gcc.gnu.org/bugzilla/show_bug.cgi?id=82624

diff -rupN gcc-7.2.0-pristine/gcc/config/msp430/msp430.c gcc-7.2.0-changed/gcc/config/msp430/msp430.c
--- gcc-7.2.0-pristine/gcc/config/msp430/msp430.c	2017-03-10 20:43:48.186872000 -0500
+++ gcc-7.2.0-changed/gcc/config/msp430/msp430.c	2017-10-18 16:00:46.677567638 -0400
@@ -748,6 +748,10 @@ hwmult_name (unsigned int val)
 static void
 msp430_option_override (void)
 {
+  /* The MSP430 architecture can safely dereference a NULL pointer. In fact,
+  there are memory mapped registers there.  */
+  flag_delete_null_pointer_checks = 0;
+
   init_machine_status = msp430_init_machine_status;
 
   if (target_cpu)
diff -rupN gcc-7.2.0-pristine/gcc/doc/g++.1 gcc-7.2.0-changed/gcc/doc/g++.1
--- gcc-7.2.0-pristine/gcc/doc/g++.1	2017-08-14 04:30:43.506125138 -0400
+++ gcc-7.2.0-changed/gcc/doc/g++.1	2017-10-18 16:07:27.160923633 -0400
@@ -7045,7 +7045,8 @@ Use \fB\-fno\-delete\-null\-pointer\-che
 for programs that depend on that behavior.
 .Sp
 This option is enabled by default on most targets.  On Nios \s-1II ELF,\s0 it
-defaults to off.  On \s-1AVR\s0 and \s-1CR16,\s0 this option is completely disabled.
+defaults to off.  On \s-1AVR\s0, \s-1CR16\s0, and \s-1MSP430\s0 this option is
+completely disabled.
 .Sp
 Passes that use the dataflow information
 are enabled independently at different optimization levels.
diff -rupN gcc-7.2.0-pristine/gcc/doc/gcc.1 gcc-7.2.0-changed/gcc/doc/gcc.1
--- gcc-7.2.0-pristine/gcc/doc/gcc.1	2017-08-14 04:30:43.002125125 -0400
+++ gcc-7.2.0-changed/gcc/doc/gcc.1	2017-10-18 16:06:31.367587143 -0400
@@ -7045,7 +7045,8 @@ Use \fB\-fno\-delete\-null\-pointer\-che
 for programs that depend on that behavior.
 .Sp
 This option is enabled by default on most targets.  On Nios \s-1II ELF,\s0 it
-defaults to off.  On \s-1AVR\s0 and \s-1CR16,\s0 this option is completely disabled.
+defaults to off.  On \s-1AVR\s0, \s-1CR16\s0, and \s-1MSP430\s0, this option is
+completely disabled.
 .Sp
 Passes that use the dataflow information
 are enabled independently at different optimization levels.
diff -rupN gcc-7.2.0-pristine/gcc/doc/gcc.info gcc-7.2.0-changed/gcc/doc/gcc.info
--- gcc-7.2.0-pristine/gcc/doc/gcc.info	2017-08-14 04:30:40.618125066 -0400
+++ gcc-7.2.0-changed/gcc/doc/gcc.info	2017-10-18 16:05:19.124249722 -0400
@@ -7070,7 +7070,7 @@ optimizations to be performed is desired
      for programs that depend on that behavior.
 
      This option is enabled by default on most targets.  On Nios II
-     ELF, it defaults to off.  On AVR and CR16, this option is
+     ELF, it defaults to off.  On AVR, CR16, and MSP430, this option is
      completely disabled.
 
      Passes that use the dataflow information are enabled independently
diff -rupN gcc-7.2.0-pristine/gcc/doc/invoke.texi gcc-7.2.0-changed/gcc/doc/invoke.texi
--- gcc-7.2.0-pristine/gcc/doc/invoke.texi	2017-07-26 08:42:03.184523000 -0400
+++ gcc-7.2.0-changed/gcc/doc/invoke.texi	2017-10-18 16:04:16.724246191 -0400
@@ -7618,7 +7618,7 @@ Use @option{-fno-delete-null-pointer-che
 for programs that depend on that behavior.
 
 This option is enabled by default on most targets.  On Nios II ELF, it
-defaults to off.  On AVR and CR16, this option is completely disabled.  
+defaults to off.  On AVR, CR16, and MSP430, this option is completely disabled.
 
 Passes that use the dataflow information
 are enabled independently at different optimization levels.
diff -rupN gcc-7.2.0-pristine/gcc/testsuite/lib/target-supports.exp gcc-7.2.0-changed/gcc/testsuite/lib/target-supports.exp
--- gcc-7.2.0-pristine/gcc/testsuite/lib/target-supports.exp	2017-03-24 09:59:51.032979000 -0400
+++ gcc-7.2.0-changed/gcc/testsuite/lib/target-supports.exp	2017-10-18 16:26:11.234320570 -0400
@@ -514,7 +514,8 @@ proc check_effective_target_keeps_null_p
     if [target_info exists keeps_null_pointer_checks] {
       return 1
     }
-    if { [istarget avr-*-*] } {
+    if { [istarget avr-*-*]
+	 || [istarget msp430-*-*] } {
 	return 1;   
     }
     return 0

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