fix Issue 24084 - Add -nothrow Switch to Compiler (#15536)

changelog/dmd.fix24084.dd

@@ -0,0 +1,21 @@
+Add -nothrow Switch to Compiler
+
+dmd itself (and presumably others) do not throw Exceptions, preferring other methods
+for dealing with errors. There is a cost, however, in supporting Exceptions even when
+they are never thrown. The cost is in adding stack unwinders for things like RAII
+objects, and preventing numerous optimizations across try-catch boundaries.
+
+Adding `nothrow` to all the code in a project turns out to be an inordinate amount of
+work if the program is large. Putting `nothrow:` at the top of the module doesn't
+influence the status for member functions in a class or struct, the `nothrow:`
+will have to be repeated for each class/struct.
+
+Adding the -nothrow switch to the compiler causes the stack unwinders to not be added
+and enables the optimizations. This capability is already there for -betterC code,
+this would just enable it for regular D code.
+
+The switch does not affect semantic analysis, just the code generation. Name mangling
+is not affected.
+
+The switch is useful for determining what effect exception handling has on an executable's
+size and performance in non-throwing code.

compiler/src/dmd/cli.d

@@ -607,6 +607,11 @@ dmd -cov -unittest myprog.d
             `Turns off all array bounds checking, even for safe functions. $(RED Deprecated
             (use $(TT $(SWLINK -boundscheck)=off) instead).)`,
         ),
+        Option("nothrow",
+            "assume no Exceptions will be thrown",
+            `Turns off generation of exception stack unwinding code, enables
+	    more efficient code for RAII objects.`,
+        ),
         Option("O",
             "optimize",
             `Optimize generated code. For fastest executables, compile

compiler/src/dmd/mars.d

@@ -1422,6 +1422,10 @@ bool parseCommandLine(const ref Strings arguments, const size_t argc, ref Param
             else
                 goto Lerror;
         }
+        else if (arg == "-nothrow") // https://dlang.org/dmd.html#switch-nothrow
+        {
+            params.useExceptions = false;
+        }
         else if (arg == "-unittest")
             params.useUnitTests = true;
         else if (p[1] == 'I')              // https://dlang.org/dmd.html#switch-I

compiler/src/dmd/statementsem.d

@@ -3281,7 +3281,7 @@ Statement statementSemanticVisit(Statement s, Scope* sc)
 
         if (!global.params.useExceptions)
         {
-            tcs.error("cannot use try-catch statements with -betterC");
+            tcs.error("cannot use try-catch statements with %s", global.params.betterC ? "-betterC".ptr : "-nothrow".ptr);
             return setError();
         }
 
@@ -3696,7 +3696,7 @@ public bool throwSemantic(const ref Loc loc, ref Expression exp, Scope* sc)
 {
     if (!global.params.useExceptions)
     {
-        loc.error("cannot use `throw` statements with -betterC");
+        loc.error("cannot use `throw` statements with %s", global.params.betterC ? "-betterC".ptr : "-nothrow".ptr);
         return false;
     }
 

compiler/test/fail_compilation/test24084.d

@@ -0,0 +1,28 @@
+/* REQUIRED_ARGS: -nothrow
+ * TEST_OUTPUT:
+---
+fail_compilation/test24084.d(110): Error: cannot use `throw` statements with -nothrow
+fail_compilation/test24084.d(112): Error: cannot use try-catch statements with -nothrow
+---
+ */
+
+// https://issues.dlang.org/show_bug.cgi?id=24084
+
+#line 100
+
+struct S
+{
+    int x;
+    ~this() { }
+}
+
+void xyzzy()
+{
+    S s;
+    throw new Exception("xx");
+
+    try
+    {
+        int y;
+    } catch (Exception) { }
+}