Don't emit init symbol for zero-initialized structs (#3131)

And optimize previous usages of it to direct memset-zero.

gen/declarations.cpp

@@ -141,12 +141,15 @@ public:
     // Skip __initZ and typeinfo for @compute device code.
     // TODO: support global variables and thus __initZ
     if (!irs->dcomputetarget) {
-      // Define the __initZ symbol.
       IrAggr *ir = getIrAggr(decl);
+
+      // Define the __initZ symbol.
+      if (!decl->zeroInit) {
         auto &initZ = ir->getInitSymbol();
         auto initGlobal = llvm::cast<LLGlobalVariable>(initZ);
         initZ = irs->setGlobalVarInitializer(initGlobal, ir->getDefaultInit());
         setLinkageAndVisibility(decl, initGlobal);
+      }
 
       // emit typeinfo
       if (!ir->suppressTypeInfo()) {

gen/llvmhelpers.cpp

@@ -1650,10 +1650,16 @@ DValue *DtoSymbolAddress(Loc &loc, Type *type, Declaration *decl) {
     IF_LOG Logger::print("Sym: type=%s\n", sdecltype->toChars());
     assert(sdecltype->ty == Tstruct);
     TypeStruct *ts = static_cast<TypeStruct *>(sdecltype);
-    assert(ts->sym);
-    DtoResolveStruct(ts->sym);
+    StructDeclaration *sd = ts->sym;
+    assert(sd);
+    DtoResolveStruct(sd);
 
-    LLValue *initsym = getIrAggr(ts->sym)->getInitSymbol();
+    if (sd->zeroInit) {
+      error(loc, "no init symbol for zero-initialized struct");
+      fatal();
+    }
+
+    LLValue *initsym = getIrAggr(sd)->getInitSymbol();
     initsym = DtoBitCast(initsym, DtoType(ts->pointerTo()));
     return new DLValue(type, initsym);
   }

gen/toir.cpp

@@ -2293,15 +2293,21 @@ public:
     LOG_SCOPE;
 
     if (e->useStaticInit) {
-      DtoResolveStruct(e->sd);
-      LLValue *initsym = getIrAggr(e->sd)->getInitSymbol();
-      initsym = DtoBitCast(initsym, DtoType(e->type->pointerTo()));
+      StructDeclaration *sd = e->sd;
+      DtoResolveStruct(sd);
 
       if (!dstMem)
         dstMem = DtoAlloca(e->type, ".structliteral");
 
+      if (sd->zeroInit) {
+        DtoMemSetZero(dstMem);
+      } else {
+        LLValue *initsym = getIrAggr(sd)->getInitSymbol();
+        initsym = DtoBitCast(initsym, DtoType(e->type->pointerTo()));
         assert(dstMem->getType() == initsym->getType());
         DtoMemCpy(dstMem, initsym);
+      }
+
       return new DLValue(e->type, dstMem);
     }
 
@@ -2739,6 +2745,26 @@ bool basetypesAreEqualWithoutModifiers(Type *l, Type *r) {
 }
 
 bool toInPlaceConstruction(DLValue *lhs, Expression *rhs) {
+  // Is the rhs the init symbol of a zero-initialized struct?
+  // Then aggressively zero-out the lhs, without any type checks, e.g., allowing
+  // to initialize a `S[1]` lhs with a `S` rhs.
+  if (auto ve = rhs->isVarExp()) {
+    if (auto symdecl = ve->var->isSymbolDeclaration()) {
+      Type *t = symdecl->type->toBasetype();
+      if (auto ts = t->isTypeStruct()) {
+        // this is the static initializer for a struct (init symbol)
+        StructDeclaration *sd = ts->sym;
+        assert(sd);
+        DtoResolveStruct(sd);
+
+        if (sd->zeroInit) {
+          DtoMemSetZero(DtoLVal(lhs));
+          return true;
+        }
+      }
+    }
+  }
+
   if (!basetypesAreEqualWithoutModifiers(lhs->type, rhs->type))
     return false;
 

tests/codegen/align.d

@@ -3,16 +3,16 @@
 // Fails on Windows_x86, see https://github.com/ldc-developers/ldc/issues/1356
 // XFAIL: Windows_x86
 
-align(32) struct Outer { int a; }
-struct Inner { align(32) int a; }
+align(32) struct Outer { int a = 1; }
+// CHECK-DAG: _D5align5Outer6__initZ = constant %align.Outer {{.*}}, align 32
+struct Inner { align(32) int a = 1; }
+// CHECK-DAG: _D5align5Inner6__initZ = constant %align.Inner {{.*}}, align 32
 
 align(1) ubyte globalByte1;
 // CHECK-DAG: _D5align11globalByte1h = {{.*}} align 1
 static Outer globalOuter;
-// CHECK-DAG: constant %align.Outer zeroinitializer{{(, comdat)?}}, align 32
 // CHECK-DAG: _D5align11globalOuterSQu5Outer = {{.*}} align 32
 static Inner globalInner;
-// CHECK-DAG: constant %align.Inner zeroinitializer{{(, comdat)?}}, align 32
 // CHECK-DAG: _D5align11globalInnerSQu5Inner = {{.*}} align 32
 
 Outer passAndReturnOuterByVal(Outer arg) { return arg; }

tests/codegen/export_aggregate_symbols.d

@@ -28,7 +28,7 @@ interface DefaultI { void foo(); }
 
 // DEFAULT:    _D24export_aggregate_symbols8DefaultS6__initZ
 // HIDDEN-NOT: _D24export_aggregate_symbols8DefaultS6__initZ
-struct DefaultS {}
+struct DefaultS { int nonZero = 1; }
 
 
 
@@ -43,7 +43,7 @@ export class ExportedC : ExportedI { void foo() {} }
 export interface ExportedI { void foo(); }
 
 // BOTH: _D24export_aggregate_symbols9ExportedS6__initZ
-export struct ExportedS {}
+export struct ExportedS { int nonZero = 1; }
 
 
 

tests/codegen/gh2235.d

@@ -1,13 +1,13 @@
 // RUN: %ldc -output-ll -of=%t.ll %s && FileCheck %s < %t.ll
 // RUN: %ldc -run %s
 
-// CHECK: %gh2235.Foo = type <{
+// CHECK-DAG: %gh2235.Foo = type <{
 align(2) struct Foo {
     long y;
     byte z;
 }
 
-// CHECK: %gh2235.Bar = type <{
+// CHECK-DAG: %gh2235.Bar = type <{
 class Bar {
     union {
         bool b;

tests/codegen/no_init_symbols_for_zeroinit_structs.d

@@ -0,0 +1,13 @@
+// RUN: %ldc -output-ll -of=%t.ll %s
+// RUN: FileCheck %s < %t.ll
+
+module mod;
+
+// CHECK-NOT: _D3mod5Empty6__initZ
+struct Empty {}
+
+// CHECK-NOT: _D3mod7WithInt6__initZ
+struct WithInt { int a; }
+
+// CHECK-NOT: _D3mod13WithZeroFloat6__initZ
+struct WithZeroFloat { float a = 0; }

tests/codegen/simd_alignment.d

@@ -15,7 +15,6 @@ struct S17237
 int4 globalIntFour;
 // CHECK-DAG: globalIntFour{{.*}} = {{.*}} align 16
 S17237 globalStruct;
-// CHECK-DAG: constant %{{.*}}.S17237 zeroinitializer{{(, comdat)?}}, align 32
 // CHECK-DAG: @{{.*}}globalStruct{{.*}}S17237{{\"?}} = {{.*}} zeroinitializer{{(, comdat)?}}, align 32
 
 // CHECK-LABEL: define <8 x i32> @foo(