fix Issue 21821 - Optimizer assumes immutables do not change, but they can in @System code

Author: WalterBright

compiler/src/dmd/backend/cc.d

@@ -749,6 +749,7 @@ enum
     Fnothrow         = 0x10000, // function does not throw (even if not marked 'nothrow')
     Feh_none         = 0x20000, // ehmethod==EH_NONE for this function only
     F3hiddenPtr      = 0x40000, // function has hidden pointer to return value
+    F3safe           = 0x80000, // function is @safe
 }
 
 struct func_t

compiler/src/dmd/backend/gdag.d

@@ -332,7 +332,7 @@ private void aewalk(elem **pn,vec_t ae)
         {
             assert(t.Eoper == OPvar);
             Symbol* s = t.EV.Vsym;
-            if (!(s.Sflags & SFLunambig))
+            if (Symbol_isAffected(*s))
                 vec_subass(ae,go.starkill);
             for (uint i = 0; (i = cast(uint) vec_index(i, ae)) < go.exptop; ++i) // for each ae elem
             {
@@ -821,7 +821,7 @@ private void abewalk(elem *n,vec_t ae,vec_t aeval)
 
             assert(t.Eoper == OPvar);
             s = t.EV.Vsym;
-            if (!(s.Sflags & SFLunambig))
+            if (Symbol_isAffected(*s))
                 vec_subass(ae,go.starkill);
             for (uint i = 0; (i = cast(uint) vec_index(i, ae)) < go.exptop; ++i) // for each ae elem
             {

compiler/src/dmd/backend/gother.d

@@ -1255,18 +1255,19 @@ private bool copyPropWalk(elem *n,vec_t IN)
                 v = go.expnod[i].EV.E1.EV.Vsym;
                 if (ambig)
                 {
-                    if (!(v.Sflags & SFLunambig))
+                    if (Symbol_isAffected(*v))
                         goto clr;
                 }
                 else
                 {
                     if (v == t.EV.Vsym)
                         goto clr;
                 }
+
                 v = go.expnod[i].EV.E2.EV.Vsym;
                 if (ambig)
                 {
-                    if (!(v.Sflags & SFLunambig))
+                    if (Symbol_isAffected(*v))
                         goto clr;
                 }
                 else

compiler/src/dmd/backend/symbol.d

@@ -282,7 +282,14 @@ bool Symbol_isAffected(const ref Symbol s)
     if (0 &&
         s.ty() & (mTYconst | mTYimmutable))
     {
-        return false;
+        /* Disabled for the moment because even @safe functions
+         * may have inlined unsafe code from other functions
+         */
+        if (funcsym_p.Sfunc.Fflags3 & F3safe &&
+            s.ty() & mTYimmutable)
+        {
+            return false;
+        }
     }
     return true;
 }

compiler/src/dmd/tocsym.d

@@ -356,6 +356,9 @@ Symbol *toSymbol(Dsymbol s)
             else if (fd.isMember2() && fd.isStatic())
                 f.Fflags |= Fstatic;
 
+            if (fd.isSafe())
+                f.Fflags3 |= F3safe;
+
             if (fd.inlining == PINLINE.default_ && global.params.useInline ||
                 fd.inlining == PINLINE.always)
             {

test/runnable/test21821.d

@@ -0,0 +1,32 @@
+// REQUIRED_ARGS: -preview=fieldwise -O
+// https://issues.dlang.org/show_bug.cgi?id=21821
+
+// test case comes from unittests in core.lifetime
+
+void test()
+{
+    alias T = immutable(S);
+    T source;
+    T target;
+    copyEmplacex(source, target);
+    T expectedCopy = source;
+    assert(target == expectedCopy);
+}
+
+struct S
+{
+    int x = 42;
+    this(this) { x += 10; }
+}
+
+void copyEmplacex(ref immutable(S) source, ref immutable(S) target) @system
+{
+        import core.stdc.string : memcpy;
+        memcpy(cast(S*) &target, cast(S*) &source, S.sizeof);
+	(cast() target).__xpostblit(); // casting away immutable
+}
+
+void main()
+{
+    test();
+}