[gcc r17-3340] AVR: Add option for strict address space subset relations.

https://gcc.gnu.org/g:cbf217f4c7076d4e23a34735d403a192d4e78191

commit r17-3340-gcbf217f4c7076d4e23a34735d403a192d4e78191
Author: Georg-Johann Lay <[email protected]>
Date:   Mon Aug 17 21:17:03 2026 +0200

    AVR: Add option for strict address space subset relations.
    
    To date, each named address space is implemented as a subset
    of all the other address spaces, which allows code like
    
       char read_char (const char *addr, bool in_flash)
       {
          return in_flash
            ? *(const __flash char*) addr  // Read from program memory.
            : *addr;                       // Read from RAM.
       }
    
    even though __flash is not a subset of Generic.
    
    This patch introduces new target option -mstrict-addr-space-subsets
    which instructs avr_addr_space_subset_p to implement the address
    space subset relations like they actually are, i.e.
    
       __memx > __flashx > __flash, __flash1, ..., __flash5
       __memx > Generic
    
    gcc/
            * config/avr/avr.opt (-mstrict-addr-space-subsets): New
            target option.
            * config/avr/avr.cc (avr_addr_space_subset_p): Use it.
            * doc/invoke.texi (AVR Options): Document it.

Diff:
---
 gcc/config/avr/avr.cc  | 22 +++++++++++-----------
 gcc/config/avr/avr.opt |  4 ++++
 gcc/doc/invoke.texi    | 19 ++++++++++++++++++-
 3 files changed, 33 insertions(+), 12 deletions(-)

diff --git a/gcc/config/avr/avr.cc b/gcc/config/avr/avr.cc
index a873a1262843..084ee2d3715a 100644
--- a/gcc/config/avr/avr.cc
+++ b/gcc/config/avr/avr.cc
@@ -15404,26 +15404,26 @@ avr_addr_space_convert (rtx src, tree type_old, tree type_new)
 }
 
 
-/* Implement `TARGET_ADDR_SPACE_SUBSET_P'.  */
+/* Helps the next two functions.  */
 
 static bool
-avr_addr_space_subset_p (addr_space_t /*subset*/, addr_space_t /*superset*/)
+avr_addr_space_contains (addr_space_t super, addr_space_t sub)
 {
-  /* Allow any kind of pointer mess.  */
-
-  return true;
+  return (super == sub
+	  || super == ADDR_SPACE_MEMX
+	  || (super == ADDR_SPACE_FLASHX
+	      && sub != ADDR_SPACE_MEMX && ! ADDR_SPACE_GENERIC_P (sub)));
 }
 
 
-/* Helps the next function.  */
+/* Implement `TARGET_ADDR_SPACE_SUBSET_P'.  */
 
 static bool
-avr_addr_space_contains (addr_space_t super, addr_space_t sub)
+avr_addr_space_subset_p (addr_space_t subset, addr_space_t superset)
 {
-  return (super == sub
-	  || super == ADDR_SPACE_MEMX
-	  || (super == ADDR_SPACE_FLASHX
-	      && sub != ADDR_SPACE_MEMX && ! ADDR_SPACE_GENERIC_P (sub)));
+  // Allow any kind of pointer casts with -mno-strict-addr-space-subsets.
+  return (!avropt_strict_addr_space_subsets
+	  || avr_addr_space_contains (superset, subset));
 }
 
 
diff --git a/gcc/config/avr/avr.opt b/gcc/config/avr/avr.opt
index 97d207726ba9..d56061b50b6c 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 2e53a8c8a7ee..d6cfc4598531 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