[patch] AVR: Support strict address space subset relations

diff --git a/gcc/config/avr/avr.cc b/gcc/config/avr/avr.cc
index 87c4044f076..7c003941bdc 100644
--- a/gcc/config/avr/avr.cc
+++ b/gcc/config/avr/avr.cc
@@ -15404,18 +15404,7 @@ avr_addr_space_convert (rtx src, tree type_old, tree type_new)
 }
 
 
-/* Implement `TARGET_ADDR_SPACE_SUBSET_P'.  */
-
-static bool
-avr_addr_space_subset_p (addr_space_t /*subset*/, addr_space_t /*superset*/)
-{
-  /* Allow any kind of pointer mess.  */
-
-  return true;
-}
-
-
-/* Helps the next function.  */
+/* Helps the next two functions.  */
 
 static bool
 avr_addr_space_contains (addr_space_t super, addr_space_t sub)
@@ -15427,6 +15416,17 @@ avr_addr_space_contains (addr_space_t super, addr_space_t sub)
 }
 
 
+/* Implement `TARGET_ADDR_SPACE_SUBSET_P'.  */
+
+static bool
+avr_addr_space_subset_p (addr_space_t subset, addr_space_t superset)
+{
+  // Allow any kind of pointer casts with -mno-strict-addr-space-subsets.
+  return (!avropt_strict_addr_space_subsets
+	  || avr_addr_space_contains (superset, subset));
+}
+
+
 /* Implement `TARGET_CONVERT_TO_TYPE'.  */
 
 static tree
diff --git a/gcc/config/avr/avr.opt b/gcc/config/avr/avr.opt
index 97d207726ba..d56061b50b6 100644
--- a/gcc/config/avr/avr.opt
+++ b/gcc/config/avr/avr.opt
@@ -115,6 +115,10 @@ msplit-ldst
 Target Var(avropt_split_ldst) Init(0) Optimization
 Optimization. Split most of the load and store instructions into byte load and stores.
 
+mstrict-addr-space-subsets
+Target Var(avropt_strict_addr_space_subsets) Init(0)
+Implement strict named address space subset relations.
+
 mstrict-X
 Target Var(avropt_strict_X) Init(0) Optimization
 Optimization. When accessing RAM, use X as imposed by the hardware, i.e. just use pre-decrement, post-increment and indirect addressing with the X register.  Without this option, the compiler may assume that there is an addressing mode X+const similar to Y+const and Z+const and emit instructions to emulate such an addressing mode for X.
diff --git a/gcc/doc/invoke.texi b/gcc/doc/invoke.texi
index 2e53a8c8a7e..d6cfc459853 100644
--- a/gcc/doc/invoke.texi
+++ b/gcc/doc/invoke.texi
@@ -991,7 +991,7 @@ Objective-C and Objective-C++ Dialects}.
 -mdouble=@var{bits}  -mlong-double=@var{bits}  -mno-call-main
 -mn_flash=@var{size}  -mfract-convert-truncate  -mno-interrupts
 -mmain-is-OS_task  -mrelax  -mpmem-wrap-around
--mrmw  -mstrict-X  -mtiny-stack
+-mrmw  -mstrict-addr-space-subsets  -mstrict-X  -mtiny-stack
 -mrodata-in-ram  -msplit-bit-shift  -msplit-ldst  -mshort-calls
 -mskip-bug  -muse-nonzero-bits  -nodevicelib  -nodevicespecs
 -Wasm-len-notes  -Waddr-space-convert  -Wmisspelled-isr}
@@ -24595,6 +24595,23 @@ aspect of the optimization.
 @item -mfuse-move2
 Run a post combine optimization pass that tries to fuse move instructions.
 
+@opindex mstrict-addr-space-subsets
+@item -mstrict-addr-space-subsets
+Impose strict named address space subset relations.  Without this option,
+every address space is considered to be a subset of all the other spaces,
+which allows to write code like
+@example
+char read_char (const char *addr, bool in_flash)
+@{
+   return in_flash
+     ? *(const __flash char*) addr  // Read from program memory.
+     : *addr;                       // Read from RAM.
+@}
+@end example
+When the option is on, such code is rejected since address space
+@code{__flash} is not a subset of the generic space, and hence the
+cast in the above code is prohibited.
+
 @opindex mstrict-X
 @item -mstrict-X
 Use address register @code{X} in a way proposed by the hardware.  This means