[patch,avr] Put a valid opcode prior to some gs() tables.

[Georg-Johann Lay]

On functional safety devices (AVR-SD), each executed instruction must
be followed by a valid opcode.  This is because instruction fetch and
decode for the next instruction runs while the 2-stage pipeline is
executing the current instruction.

There is only one case where avr-gcc generates code interspersed with
data, which is when a switch/case table is generated for a function
with a "section" attribute and AVR_HAVE_JMP_CALL.  In that case, the
table with the gs() code label addresses is put in .subsection 1 so
that it belongs to the section as specified by the "section" attribute.

Ok to apply?

Johann

--

AVR: AVR-SD: Put a valid opcode prior to gs() table in .subsection 1.

On functional safety devices (AVR-SD), each executed instruction must
be followed by a valid opcode.  This is because instruction fetch and
decode for the next instruction runs while the 2-stage pipeline is
executing the current instruction.

There is only one case where avr-gcc generates code interspersed with
data, which is when a switch/case table is generated for a function
with a "section" attribute and AVR_HAVE_JMP_CALL.  In that case, the
table with the gs() code label addresses is put in .subsection 1 so
that it belongs to the section as specified by the "section" attribute.

gcc/
        * gcc/config/avr/avr.cc (avr_output_addr_vec): Output
        a valid opcode prior to the first gs() label provided:
        - The code is compiled for an arch that has AVR-SD mcus, and
        - the function has a "section" attribute, and
        - the function has a gs() label addresses switch/case table.
diff --git a/gcc/config/avr/avr.cc b/gcc/config/avr/avr.cc
index 0bdba554575..c42621a54c0 100644
--- a/gcc/config/avr/avr.cc
+++ b/gcc/config/avr/avr.cc
@@ -14394,6 +14394,16 @@ avr_output_addr_vec (rtx_insn *labl, rtx table)
 {
   FILE *stream = asm_out_file;
 
+  // AVR-SD: On functional safety devices, each executed instruction must
+  // be followd by a valid opcode.  This is because instruction fetch
+  // and decode for the next instruction runs while the 2-stage pipeline
+  // is executing the current instruction.  There is no multilib option for
+  // these devices, so take all multilib variants that contain AVR-SD.
+  const bool maybe_sd = (AVR_HAVE_JMP_CALL
+			 && (avr_arch_index == ARCH_AVRXMEGA2
+			     || avr_arch_index == ARCH_AVRXMEGA3));
+  bool uses_subsection = false;
+
   app_disable ();
 
   // Switch to appropriate (sub)section.
@@ -14407,6 +14417,7 @@ avr_output_addr_vec (rtx_insn *labl, rtx table)
 
       switch_to_section (current_function_section ());
       fprintf (stream, "\t.subsection\t1\n");
+      uses_subsection = true;
     }
   else
     {
@@ -14429,10 +14440,21 @@ avr_output_addr_vec (rtx_insn *labl, rtx table)
 	       AVR_HAVE_JMP_CALL ? "a" : "ax");
     }
 
-  // Output the label that precedes the table.
-
   ASM_OUTPUT_ALIGN (stream, 1);
 
+  if (maybe_sd && uses_subsection)
+    {
+      // Insert a valid opcode prior to the first gs() label.
+      // Any valid opcode will do.  Use CLH since it disassembles
+      // more nicely than NOP = 0x0000.  This is all GCC can do.
+      // Other cases, like inserting CLH after the vector table and
+      // after the last instruction, are handled by other parts of
+      // the toolchain.
+      fprintf (stream, "\tclh\n");
+    }
+
+  // Output the label that precedes the table.
+
   char s_labl[40];
   targetm.asm_out.generate_internal_label (s_labl, "L",
 					   CODE_LABEL_NUMBER (labl));