[CIR][CodeGen] handle zero init padding case (#1257)

I continue to use `csmith` and catch run time bags. Now it's time to fix
the layout for the const structs.

There is a divergence between const structs generated by CIR and the
original codegen. And this PR makes one more step to eliminate it. There
are cases where the extra padding is required - and here is a fix for
some of them. I did not write extra tests, since the fixes in the
existing already covers the code I added. The point is that now the
layout for all of these structs in the LLVM IR with and without CIR is
the same.

clang/lib/CIR/CodeGen/CIRGenExprConst.cpp

@@ -43,6 +43,19 @@ using namespace clang::CIRGen;
 namespace {
 class ConstExprEmitter;
 
+static mlir::TypedAttr computePadding(CIRGenModule &CGM, CharUnits size) {
+  auto eltTy = CGM.UCharTy;
+  auto arSize = size.getQuantity();
+  auto &bld = CGM.getBuilder();
+  if (size > CharUnits::One()) {
+    SmallVector<mlir::Attribute, 4> elts(arSize, bld.getZeroAttr(eltTy));
+    return bld.getConstArray(mlir::ArrayAttr::get(bld.getContext(), elts),
+                             bld.getArrayType(eltTy, arSize));
+  } else {
+    return cir::ZeroAttr::get(bld.getContext(), eltTy);
+  }
+}
+
 static mlir::Attribute
 emitArrayConstant(CIRGenModule &CGM, mlir::Type DesiredType,
                   mlir::Type CommonElementType, unsigned ArrayBound,
@@ -70,12 +83,7 @@ struct ConstantAggregateBuilderUtils {
   }
 
   mlir::TypedAttr getPadding(CharUnits size) const {
-    auto eltTy = CGM.UCharTy;
-    auto arSize = size.getQuantity();
-    auto &bld = CGM.getBuilder();
-    SmallVector<mlir::Attribute, 4> elts(arSize, bld.getZeroAttr(eltTy));
-    return bld.getConstArray(mlir::ArrayAttr::get(bld.getContext(), elts),
-                             bld.getArrayType(eltTy, arSize));
+    return computePadding(CGM, size);
   }
 
   mlir::Attribute getZeroes(CharUnits ZeroSize) const {
@@ -508,6 +516,11 @@ class ConstStructBuilder {
   bool Build(InitListExpr *ILE, bool AllowOverwrite);
   bool Build(const APValue &Val, const RecordDecl *RD, bool IsPrimaryBase,
              const CXXRecordDecl *VTableClass, CharUnits BaseOffset);
+
+  bool ApplyZeroInitPadding(const ASTRecordLayout &Layout, unsigned FieldNo,
+                            const FieldDecl &Field, bool AllowOverwrite,
+                            CharUnits &SizeSoFar, bool &ZeroFieldSize);
+
   mlir::Attribute Finalize(QualType Ty);
 };
 
@@ -614,6 +627,10 @@ bool ConstStructBuilder::Build(InitListExpr *ILE, bool AllowOverwrite) {
     if (CXXRD->getNumBases())
       return false;
 
+  const bool ZeroInitPadding = CGM.shouldZeroInitPadding();
+  bool ZeroFieldSize = false;
+  CharUnits SizeSoFar = CharUnits::Zero();
+
   for (FieldDecl *Field : RD->fields()) {
     ++FieldNo;
 
@@ -642,6 +659,11 @@ bool ConstStructBuilder::Build(InitListExpr *ILE, bool AllowOverwrite) {
       continue;
     }
 
+    if (ZeroInitPadding &&
+        !ApplyZeroInitPadding(Layout, FieldNo, *Field, AllowOverwrite,
+                              SizeSoFar, ZeroFieldSize))
+      return false;
+
     // When emitting a DesignatedInitUpdateExpr, a nested InitListExpr
     // represents additional overwriting of our current constant value, and not
     // a new constant to emit independently.
@@ -784,6 +806,38 @@ bool ConstStructBuilder::Build(const APValue &Val, const RecordDecl *RD,
   return true;
 }
 
+bool ConstStructBuilder::ApplyZeroInitPadding(
+    const ASTRecordLayout &Layout, unsigned FieldNo, const FieldDecl &Field,
+    bool AllowOverwrite, CharUnits &SizeSoFar, bool &ZeroFieldSize) {
+
+  uint64_t StartBitOffset = Layout.getFieldOffset(FieldNo);
+  CharUnits StartOffset =
+      CGM.getASTContext().toCharUnitsFromBits(StartBitOffset);
+  if (SizeSoFar < StartOffset) {
+    if (!AppendBytes(SizeSoFar, computePadding(CGM, StartOffset - SizeSoFar),
+                     AllowOverwrite))
+      return false;
+  }
+
+  if (!Field.isBitField()) {
+    CharUnits FieldSize =
+        CGM.getASTContext().getTypeSizeInChars(Field.getType());
+    SizeSoFar = StartOffset + FieldSize;
+    ZeroFieldSize = FieldSize.isZero();
+  } else {
+    const CIRGenRecordLayout &RL =
+        CGM.getTypes().getCIRGenRecordLayout(Field.getParent());
+    const CIRGenBitFieldInfo &Info = RL.getBitFieldInfo(&Field);
+    uint64_t EndBitOffset = StartBitOffset + Info.Size;
+    SizeSoFar = CGM.getASTContext().toCharUnitsFromBits(EndBitOffset);
+    if (EndBitOffset % CGM.getASTContext().getCharWidth() != 0) {
+      SizeSoFar++;
+    }
+    ZeroFieldSize = Info.Size == 0;
+  }
+  return true;
+}
+
 mlir::Attribute ConstStructBuilder::Finalize(QualType Type) {
   Type = Type.getNonReferenceType();
   RecordDecl *RD = Type->castAs<RecordType>()->getDecl();

clang/lib/CIR/CodeGen/CIRGenModule.h

@@ -273,6 +273,56 @@ class CIRGenModule : public CIRGenTypeCache {
     return getTriple().isSPIRVLogical();
   }
 
+  bool shouldZeroInitPadding() const {
+    // In C23 (N3096) $6.7.10:
+    // """
+    // If any object is initialized with an empty initializer, then it is
+    // subject to default initialization:
+    //  - if it is an aggregate, every member is initialized (recursively)
+    //  according to these rules, and any padding is initialized to zero bits;
+    //  - if it is a union, the first named member is initialized (recursively)
+    //  according to these rules, and any padding is initialized to zero bits.
+    //
+    // If the aggregate or union contains elements or members that are
+    // aggregates or unions, these rules apply recursively to the subaggregates
+    // or contained unions.
+    //
+    // If there are fewer initializers in a brace-enclosed list than there are
+    // elements or members of an aggregate, or fewer characters in a string
+    // literal used to initialize an array of known size than there are elements
+    // in the array, the remainder of the aggregate is subject to default
+    // initialization.
+    // """
+    //
+    // The standard seems ambiguous in the following two areas:
+    // 1. For a union type with empty initializer, if the first named member is
+    // not the largest member, then the bytes comes after the first named member
+    // but before padding are left unspecified. An example is:
+    //    union U { int a; long long b;};
+    //    union U u = {};  // The first 4 bytes are 0, but 4-8 bytes are left
+    //    unspecified.
+    //
+    // 2. It only mentions padding for empty initializer, but doesn't mention
+    // padding for a non empty initialization list. And if the aggregation or
+    // union contains elements or members that are aggregates or unions, and
+    // some are non empty initializers, while others are empty initializers,
+    // the padding initialization is unclear. An example is:
+    //    struct S1 { int a; long long b; };
+    //    struct S2 { char c; struct S1 s1; };
+    //    // The values for paddings between s2.c and s2.s1.a, between s2.s1.a
+    //    and s2.s1.b are unclear.
+    //    struct S2 s2 = { 'c' };
+    //
+    // Here we choose to zero initiailize left bytes of a union type because
+    // projects like the Linux kernel are relying on this behavior. If we don't
+    // explicitly zero initialize them, the undef values can be optimized to
+    // return garbage data. We also choose to zero initialize paddings for
+    // aggregates and unions, no matter they are initialized by empty
+    // initializers or non empty initializers. This can provide a consistent
+    // behavior. So projects like the Linux kernel can rely on it.
+    return !getLangOpts().CPlusPlus;
+  }
+
   /// Return the mlir::Value for the address of the given global variable.
   /// If Ty is non-null and if the global doesn't exist, then it will be created
   /// with the specified type instead of whatever the normal requested type

clang/test/CIR/CodeGen/bitfields.c

@@ -59,12 +59,12 @@ typedef struct {
 // CHECK: !ty_G = !cir.struct<struct "G" {!u16i, !s32i} #cir.record.decl.ast>
 // CHECK: !ty_T = !cir.struct<struct "T" {!u8i, !u32i} #cir.record.decl.ast>
 // CHECK: !ty_anon2E0_ = !cir.struct<struct "anon.0" {!u32i} #cir.record.decl.ast>
-// CHECK: !ty_anon_struct = !cir.struct<struct  {!u8i, !u8i, !s32i}>
 // CHECK: #bfi_a = #cir.bitfield_info<name = "a", storage_type = !u8i, size = 3, offset = 0, is_signed = true>
 // CHECK: #bfi_e = #cir.bitfield_info<name = "e", storage_type = !u16i, size = 15, offset = 0, is_signed = true>
 // CHECK: !ty_S = !cir.struct<struct "S" {!u32i, !cir.array<!u8i x 3>, !u16i, !u32i}>
 // CHECK: !ty_U = !cir.struct<struct "U" {!s8i, !s8i, !s8i, !cir.array<!u8i x 9>}>
 // CHECK: !ty___long = !cir.struct<struct "__long" {!ty_anon2E0_, !u32i, !cir.ptr<!u32i>}>
+// CHECK: !ty_anon_struct = !cir.struct<struct  {!u8i, !u8i, !cir.array<!u8i x 2>, !s32i}>
 // CHECK: #bfi_d = #cir.bitfield_info<name = "d", storage_type = !cir.array<!u8i x 3>, size = 2, offset = 17, is_signed = true>
 
 // CHECK: cir.func {{.*@store_field}}
@@ -127,7 +127,7 @@ void createU() {
 // CHECK: cir.func {{.*@createD}}
 // CHECK:   %0 = cir.alloca !ty_D, !cir.ptr<!ty_D>, ["d"] {alignment = 4 : i64}
 // CHECK:   %1 = cir.cast(bitcast, %0 : !cir.ptr<!ty_D>), !cir.ptr<!ty_anon_struct>
-// CHECK:   %2 = cir.const #cir.const_struct<{#cir.int<33> : !u8i, #cir.int<0> : !u8i, #cir.int<3> : !s32i}> : !ty_anon_struct
+// CHECK:   %2 = cir.const #cir.const_struct<{#cir.int<33> : !u8i, #cir.int<0> : !u8i, #cir.const_array<[#cir.zero : !u8i, #cir.zero : !u8i]> : !cir.array<!u8i x 2>, #cir.int<3> : !s32i}> : !ty_anon_struct
 // CHECK:   cir.store %2, %1 : !ty_anon_struct, !cir.ptr<!ty_anon_struct>
 void createD() {
   D d = {1,2,3};
@@ -148,7 +148,7 @@ typedef struct {
   int y ;
 } G;
 
-// CHECK: cir.global external @g = #cir.const_struct<{#cir.int<133> : !u8i, #cir.int<127> : !u8i, #cir.int<254> : !s32i}> : !ty_anon_struct 
+// CHECK: cir.global external @g = #cir.const_struct<{#cir.int<133> : !u8i, #cir.int<127> : !u8i, #cir.const_array<[#cir.zero : !u8i, #cir.zero : !u8i]> : !cir.array<!u8i x 2>, #cir.int<254> : !s32i}> : !ty_anon_struct
 G g = { -123, 254UL};
 
 // CHECK: cir.func {{.*@get_y}}

clang/test/CIR/CodeGen/const-bitfields.c

@@ -14,13 +14,13 @@ struct Inner {
   unsigned d : 30;
 };
 
-// CHECK: !ty_anon_struct = !cir.struct<struct  {!u8i, !u8i, !u8i, !s32i}>
+// CHECK: !ty_anon_struct = !cir.struct<struct  {!u8i, !u8i, !u8i, !u8i, !s32i}>
 // CHECK: !ty_T = !cir.struct<struct "T" {!cir.array<!u8i x 3>, !s32i} #cir.record.decl.ast>
 // CHECK: !ty_anon_struct1 = !cir.struct<struct  {!u8i, !cir.array<!u8i x 3>, !u8i, !u8i, !u8i, !u8i}>
 // CHECK: #bfi_Z = #cir.bitfield_info<name = "Z", storage_type = !cir.array<!u8i x 3>, size = 9, offset = 11, is_signed = true>
 
 struct T GV = { 1, 5, 26, 42 };
-// CHECK: cir.global external @GV = #cir.const_struct<{#cir.int<161> : !u8i, #cir.int<208> : !u8i, #cir.int<0> : !u8i, #cir.int<42> : !s32i}> : !ty_anon_struct
+// CHECK: cir.global external @GV = #cir.const_struct<{#cir.int<161> : !u8i, #cir.int<208> : !u8i, #cir.int<0> : !u8i,  #cir.zero : !u8i, #cir.int<42> : !s32i}> : !ty_anon_struct
 
 // check padding is used (const array of zeros)
 struct Inner var = { 1, 0, 1, 21};

clang/test/CIR/CodeGen/struct.c

@@ -38,8 +38,9 @@ void shouldConstInitStructs(void) {
 // CHECK: cir.func @shouldConstInitStructs
   struct Foo f = {1, 2, {3, 4}};
   // CHECK: %[[#V0:]] = cir.alloca !ty_Foo, !cir.ptr<!ty_Foo>, ["f"] {alignment = 4 : i64}
-  // CHECK: %[[#V1:]] = cir.const #cir.const_struct<{#cir.int<1> : !s32i, #cir.int<2> : !s8i, #cir.const_struct<{#cir.int<3> : !s32i, #cir.int<4> : !s8i}> : !ty_Bar}> : !ty_Foo
-  // CHECK: cir.store %[[#V1]], %[[#V0]] : !ty_Foo, !cir.ptr<!ty_Foo>
+  // CHECK: %[[#V1:]] = cir.cast(bitcast, %[[#V0]] : !cir.ptr<!ty_Foo>), !cir.ptr<!ty_anon_struct> l
+  // CHECK: %[[#V2:]] = cir.const #cir.const_struct<{#cir.int<1> : !s32i, #cir.int<2> : !s8i, #cir.const_array<[#cir.zero : !u8i, #cir.zero : !u8i, #cir.zero : !u8i]> : !cir.array<!u8i x 3>, #cir.const_struct<{#cir.int<3> : !s32i, #cir.int<4> : !s8i}> : !ty_Bar}> : !ty_anon_struct
+  // CHECK: cir.store %[[#V2]], %[[#V1]] : !ty_anon_struct, !cir.ptr<!ty_anon_struct>
 }
 
 // Should zero-initialize uninitialized global structs.