Don't try to follow non-const symbols

If a symbol is not a constant variable then its value may change. As a
result, we cannot reason about its value and hence we resort to the old
mechanism of all type compatible functions. The only exception is when
the symbol is the function itself which is implictly const

As we now require the symbol to be const, updated the tests to reflect
this. Adding tests for demonstrating what was the problem with non constant
pointers

Author: thk123

regression/goto-analyzer/fp-removal1/main.c

@@ -18,7 +18,7 @@ const void_fp fp_all[] = {f1, f2 ,f3, f4, f5 ,f6, f7, f8, f9};
 
 void func()
 {
-  void_fp fp = f2;
+  const void_fp fp = f2;
   fp();
 }
 

regression/goto-analyzer/fp-removal15/main.c

@@ -0,0 +1,28 @@
+#include <stdio.h>
+
+void f1 (void) { printf("%i", 1); }
+void f2 (void) { printf("%i", 2); }
+void f3 (void) { printf("%i", 3); }
+void f4 (void) { printf("%i", 4); }
+void f5 (void) { printf("%i", 5); }
+void f6 (void) { printf("%i", 6); }
+void f7 (void) { printf("%i", 7); }
+void f8 (void) { printf("%i", 8); }
+void f9 (void) { printf("%i", 9); }
+
+typedef void(*void_fp)(void);
+
+// There is a basic check that excludes all functions that aren't used anywhere
+// This ensures that check can't work in this example
+const void_fp fp_all[] = {f1, f2 ,f3, f4, f5 ,f6, f7, f8, f9};
+
+void func()
+{
+  void_fp fp = f2;
+  fp = f3;
+  fp();
+}
+
+void main(){
+  func();
+}

regression/goto-analyzer/fp-removal15/test.desc

@@ -0,0 +1,17 @@
+CORE
+main.c
+--show-goto-functions
+
+^Removing function pointers and virtual functions$
+^\s*IF fp == f1 THEN GOTO 1$
+^\s*IF fp == f2 THEN GOTO 2$
+^\s*IF fp == f3 THEN GOTO 3$
+^\s*IF fp == f4 THEN GOTO 4$
+^\s*IF fp == f5 THEN GOTO 5$
+^\s*IF fp == f6 THEN GOTO 6$
+^\s*IF fp == f7 THEN GOTO 7$
+^\s*IF fp == f8 THEN GOTO 8$
+^\s*IF fp == f9 THEN GOTO 9$
+^SIGNAL=0$
+--
+^warning: ignoring

regression/goto-analyzer/fp-removal16/main.c

@@ -0,0 +1,31 @@
+#include <stdio.h>
+
+void f1 (void) { printf("%i", 1); }
+void f2 (void) { printf("%i", 2); }
+void f3 (void) { printf("%i", 3); }
+void f4 (void) { printf("%i", 4); }
+void f5 (void) { printf("%i", 5); }
+void f6 (void) { printf("%i", 6); }
+void f7 (void) { printf("%i", 7); }
+void f8 (void) { printf("%i", 8); }
+void f9 (void) { printf("%i", 9); }
+
+typedef void(*void_fp)(void);
+
+// There is a basic check that excludes all functions that aren't used anywhere
+// This ensures that check can't work in this example
+const void_fp fp_all[] = {f1, f2 ,f3, f4, f5 ,f6, f7, f8, f9};
+
+void func()
+{
+  const void_fp fp = f2;
+  const void_fp fp3 = f4;
+  void_fp fp2 = fp;
+  fp2 = fp3;
+  fp2();
+}
+
+void main()
+{
+  func();
+}

regression/goto-analyzer/fp-removal16/test.desc

@@ -0,0 +1,16 @@
+CORE
+main.c
+--show-goto-functions
+^Removing function pointers and virtual functions$
+^\s*IF fp2 == f1 THEN GOTO 1$
+^\s*IF fp2 == f2 THEN GOTO 2$
+^\s*IF fp2 == f3 THEN GOTO 3$
+^\s*IF fp2 == f4 THEN GOTO 4$
+^\s*IF fp2 == f5 THEN GOTO 5$
+^\s*IF fp2 == f6 THEN GOTO 6$
+^\s*IF fp2 == f7 THEN GOTO 7$
+^\s*IF fp2 == f8 THEN GOTO 8$
+^\s*IF fp2 == f9 THEN GOTO 9$
+^SIGNAL=0$
+--
+^warning: ignoring

regression/goto-analyzer/fp-removal2/main.c

@@ -19,7 +19,7 @@ const void_fp fp_all[] = {f1, f2 ,f3, f4, f5 ,f6, f7, f8, f9};
 void func()
 {
   void_fp fp = f2;
-  void_fp fp2 = fp;
+  const void_fp fp2 = fp;
   fp2();
 }
 

regression/goto-analyzer/fp-removal3/main.c

@@ -20,7 +20,7 @@ const void_fp fp_all[] = {f1, f2 ,f3, f4, f5 ,f6, f7, f8, f9};
 
 void func()
 {
-  void_fp fp = fp_tbl[1];
+  const void_fp fp = fp_tbl[1];
   fp();
 }
 

regression/goto-analyzer/fp-removal4/main.c

@@ -21,7 +21,7 @@ const void_fp fp_all[] = {f1, f2 ,f3, f4, f5 ,f6, f7, f8, f9};
 void func()
 {
   int x = 1;
-  void_fp fp = fp_tbl[x];
+  const void_fp fp = fp_tbl[x];
   fp();
 }
 

regression/goto-analyzer/fp-removal5/main.c

@@ -17,7 +17,7 @@ const void_fp fp_tbl[] = {f2, f3 ,f4};
 const void_fp fp_all[] = {f1, f2 ,f3, f4, f5 ,f6, f7, f8, f9};
 
 void func(int i){
-   void_fp fp = fp_tbl[i];
+   const void_fp fp = fp_tbl[i];
    fp();
 }
 

src/goto-programs/remove_function_pointers.cpp

@@ -706,8 +706,19 @@ void remove_function_pointerst::remove_function_pointer(
     to_code_function_call(target->code).function()=precise_call;
     return;
   }
+
+  const c_qualifierst pointer_qualifers(pointer.type());
+
   found_functions=found_functions||try_get_from_address_of(pointer, functions);
-  found_functions=found_functions||try_get_call_from_symbol(pointer, functions);
+
+  // If it is a symbol (except in the case where the symbol is the function
+  // symbol itself) then the symbol must be const or else can be reassigned.
+  if(pointer_qualifers.is_constant)
+  {
+    found_functions=
+      found_functions||try_get_call_from_symbol(pointer, functions);
+  }
+
   found_functions=found_functions||try_get_call_from_index(pointer, functions);
 
   if(functions.size()==1)