AVR: target/121277 - Don't load 0x800000 with const __flashx *x = NULL.

Converting from generic AS to __flashx used the same rule like
for __memx, which tags RAM (generic AS) locations by setting bit 23.
The justification was that generic isn't a subset of __flashx, though
that lead to surprises with code like const __flashx *x = NULL.

The natural thing to do is to just load 0x000000 in that case,
so that the null pointer works in __flashx as expected.

Apart from that, converting NULL to __flashx (or __flash) no more
raises a -Waddr-space-convert diagnostic.

gcc/
	PR target/121277
	* config/avr/avr.cc (avr_addr_space_convert): When converting
	from generic AS to __flashx, don't set bit 23.
	(avr_convert_to_type): Don't -Waddr-space-convert when NULL
	is converted to __flashx or to __flash.

(cherry picked from commit 089faf54fa96565784ebdd8dfcf9c350c4c3bee5)

diff --git a/gcc/config/avr/avr.cc b/gcc/config/avr/avr.cc
index b192a12671f062701a2cee5702ca2063270c44f0..6b5826672849787f2ed183718af7ba33deb2fce0 100644 (file)
--- a/gcc/config/avr/avr.cc
+++ b/gcc/config/avr/avr.cc
@@ -14988,10 +14988,11 @@ avr_addr_space_convert (rtx src, tree type_old, tree type_new)
 
       /* Linearize memory: RAM has bit 23 set.  When as_new = __flashx then
         this is basically UB since __flashx mistreats RAM addresses, but there
-        is no way to bail out.  (Though -Waddr-space-convert will tell.)  */
+        is no way to bail out.  (Though -Waddr-space-convert will tell.)
+        ...but PR121277 is confusing, in particular when NULL is coming in. */
 
       int msb = ADDR_SPACE_GENERIC_P (as_old)
-       ? 0x80
+       ? as_new == ADDR_SPACE_MEMX ? 0x80 : 0x00
        : avr_addrspace[as_old].segment;
 
       src = force_reg (Pmode, src);
@@ -15089,10 +15090,16 @@ avr_convert_to_type (tree type, tree expr)
          const char *name_old = avr_addrspace[as_old].name;
          const char *name_new = avr_addrspace[as_new].name;
 
-         warning (OPT_Waddr_space_convert,
-                  "conversion from address space %qs to address space %qs",
-                  ADDR_SPACE_GENERIC_P (as_old) ? "generic" : name_old,
-                  ADDR_SPACE_GENERIC_P (as_new) ? "generic" : name_new);
+         // Be relaxed when NULL is used, and when 0x0 stands for
+         // address 0x0.
+         bool nowarn = (expr == null_pointer_node
+                        && (as_new == ADDR_SPACE_FLASHX
+                            || as_new == ADDR_SPACE_FLASH));
+         if (!nowarn)
+           warning (OPT_Waddr_space_convert,
+                    "conversion from address space %qs to address space %qs",
+                    ADDR_SPACE_GENERIC_P (as_old) ? "generic" : name_old,
+                    ADDR_SPACE_GENERIC_P (as_new) ? "generic" : name_new);
 
          return fold_build1_loc (loc, ADDR_SPACE_CONVERT_EXPR, type, expr);
        }