[clang][bytecode] Pass (float) BitWidth to DoBitCast (#119119)

In certain cases (i.e. long double on x86), the bit with we get from the
floating point semantics is different than the type size we compute for
the BitCast instruction. Pass this along to DoBitCast, so in there we
can check only the relevant bits for being initialized.

This also fixes a weirdness we still had in DoBitCast.

Author: tbaederr

clang/lib/AST/ByteCode/BitcastBuffer.h

@@ -45,6 +45,7 @@ struct Bits {
   bool operator>=(Bits Other) const { return N >= Other.N; }
   bool operator<=(Bits Other) const { return N <= Other.N; }
   bool operator==(Bits Other) const { return N == Other.N; }
+  bool operator!=(Bits Other) const { return N != Other.N; }
 };
 
 /// A quantity in bytes.

clang/lib/AST/ByteCode/Interp.h

@@ -14,6 +14,7 @@
 #define LLVM_CLANG_AST_INTERP_INTERP_H
 
 #include "../ExprConstShared.h"
+#include "BitcastBuffer.h"
 #include "Boolean.h"
 #include "DynamicAllocator.h"
 #include "FixedPoint.h"
@@ -3050,24 +3051,24 @@ inline bool BitCast(InterpState &S, CodePtr OpPC, bool TargetIsUCharOrByte,
   llvm::SmallVector<std::byte> Buff(BuffSize);
   bool HasIndeterminateBits = false;
 
-  if (!DoBitCast(S, OpPC, FromPtr, Buff.data(), BuffSize, HasIndeterminateBits))
+  Bits FullBitWidth(ResultBitWidth);
+  Bits BitWidth = FullBitWidth;
+
+  if constexpr (std::is_same_v<T, Floating>) {
+    assert(Sem);
+    BitWidth = Bits(llvm::APFloatBase::getSizeInBits(*Sem));
+  }
+
+  if (!DoBitCast(S, OpPC, FromPtr, Buff.data(), BitWidth, FullBitWidth,
+                 HasIndeterminateBits))
     return false;
 
   if (!CheckBitCast(S, OpPC, HasIndeterminateBits, TargetIsUCharOrByte))
     return false;
 
   if constexpr (std::is_same_v<T, Floating>) {
     assert(Sem);
-    ptrdiff_t Offset = 0;
-
-    if (llvm::sys::IsBigEndianHost) {
-      unsigned NumBits = llvm::APFloatBase::getSizeInBits(*Sem);
-      assert(NumBits % 8 == 0);
-      assert(NumBits <= ResultBitWidth);
-      Offset = (ResultBitWidth - NumBits) / 8;
-    }
-
-    S.Stk.push<Floating>(T::bitcastFromMemory(Buff.data() + Offset, *Sem));
+    S.Stk.push<Floating>(T::bitcastFromMemory(Buff.data(), *Sem));
   } else {
     assert(!Sem);
     S.Stk.push<T>(T::bitcastFromMemory(Buff.data(), ResultBitWidth));

clang/lib/AST/ByteCode/InterpBuiltinBitCast.cpp

@@ -278,44 +278,49 @@ static bool readPointerToBuffer(const Context &Ctx, const Pointer &FromPtr,
           if (llvm::sys::IsBigEndianHost)
             swapBytes(Buff.get(), NumBits.roundToBytes());
 
+          Buffer.markInitialized(BitOffset, NumBits);
         } else {
           BITCAST_TYPE_SWITCH(T, { P.deref<T>().bitcastToMemory(Buff.get()); });
 
           if (llvm::sys::IsBigEndianHost)
             swapBytes(Buff.get(), FullBitWidth.roundToBytes());
+          Buffer.markInitialized(BitOffset, BitWidth);
         }
 
         Buffer.pushData(Buff.get(), BitOffset, BitWidth, TargetEndianness);
-        Buffer.markInitialized(BitOffset, BitWidth);
         return true;
       });
 }
 
 bool clang::interp::DoBitCast(InterpState &S, CodePtr OpPC, const Pointer &Ptr,
-                              std::byte *Buff, size_t BuffSize,
+                              std::byte *Buff, Bits BitWidth, Bits FullBitWidth,
                               bool &HasIndeterminateBits) {
   assert(Ptr.isLive());
   assert(Ptr.isBlockPointer());
   assert(Buff);
+  assert(BitWidth <= FullBitWidth);
+  assert(FullBitWidth.isFullByte());
+  assert(BitWidth.isFullByte());
 
-  Bits BitSize = Bytes(BuffSize).toBits();
-  BitcastBuffer Buffer(BitSize);
+  BitcastBuffer Buffer(FullBitWidth);
+  size_t BuffSize = FullBitWidth.roundToBytes();
   if (!CheckBitcastType(S, OpPC, Ptr.getType(), /*IsToType=*/false))
     return false;
 
   bool Success = readPointerToBuffer(S.getContext(), Ptr, Buffer,
                                      /*ReturnOnUninit=*/false);
-  HasIndeterminateBits = !Buffer.allInitialized();
+  HasIndeterminateBits = !Buffer.rangeInitialized(Bits::zero(), BitWidth);
 
   const ASTContext &ASTCtx = S.getASTContext();
   Endian TargetEndianness =
       ASTCtx.getTargetInfo().isLittleEndian() ? Endian::Little : Endian::Big;
-  auto B = Buffer.copyBits(Bits::zero(), BitSize, BitSize, TargetEndianness);
+  auto B =
+      Buffer.copyBits(Bits::zero(), BitWidth, FullBitWidth, TargetEndianness);
 
   std::memcpy(Buff, B.get(), BuffSize);
 
   if (llvm::sys::IsBigEndianHost)
-    swapBytes(Buff, BuffSize);
+    swapBytes(Buff, BitWidth.roundToBytes());
 
   return Success;
 }

clang/lib/AST/ByteCode/InterpBuiltinBitCast.h

@@ -9,6 +9,7 @@
 #ifndef LLVM_CLANG_AST_INTERP_BUILITN_BIT_CAST_H
 #define LLVM_CLANG_AST_INTERP_BUILITN_BIT_CAST_H
 
+#include "BitcastBuffer.h"
 #include <cstddef>
 
 namespace clang {
@@ -18,7 +19,8 @@ class InterpState;
 class CodePtr;
 
 bool DoBitCast(InterpState &S, CodePtr OpPC, const Pointer &Ptr,
-               std::byte *Buff, size_t BuffSize, bool &HasIndeterminateBits);
+               std::byte *Buff, Bits BitWidth, Bits FullBitWidth,
+               bool &HasIndeterminateBits);
 bool DoBitCastPtr(InterpState &S, CodePtr OpPC, const Pointer &FromPtr,
                   Pointer &ToPtr);
 bool DoBitCastPtr(InterpState &S, CodePtr OpPC, const Pointer &FromPtr,

clang/test/AST/ByteCode/builtin-bit-cast-long-double.cpp

@@ -6,7 +6,10 @@
 // RUN: %clang_cc1 -verify=expected,both -std=c++2a -fsyntax-only -triple x86_64-apple-macosx10.14.0 %s -fno-signed-char -fexperimental-new-constant-interpreter
 // RUN: %clang_cc1 -verify=expected,both -std=c++2a -fsyntax-only -triple aarch64_be-linux-gnu %s -fexperimental-new-constant-interpreter
 
+#if !__x86_64
 // both-no-diagnostics
+#endif
+
 
 typedef decltype(nullptr) nullptr_t;
 typedef __INTPTR_TYPE__ intptr_t;
@@ -35,6 +38,7 @@ constexpr Init round_trip(const Init &init) {
 
 namespace test_long_double {
 #if __x86_64
+/// FIXME: We could enable this, but since it aborts, it causes the usual mempory leak.
 #if 0
 constexpr __int128_t test_cast_to_int128 = bit_cast<__int128_t>((long double)0); // expected-error{{must be initialized by a constant expression}}\
                                                                                  // expected-note{{in call}}
@@ -49,7 +53,13 @@ static_assert(round_trip<bytes>(ld), "");
 
 static_assert(round_trip<long double>(10.0L));
 
-#if 0
+constexpr long double foo() {
+  bytes A = __builtin_bit_cast(bytes, ld);
+  long double ld = __builtin_bit_cast(long double, A);
+  return ld;
+}
+static_assert(foo() == ld);
+
 constexpr bool f(bool read_uninit) {
   bytes b = bit_cast<bytes>(ld);
   unsigned char ld_bytes[10] = {
@@ -61,16 +71,15 @@ constexpr bool f(bool read_uninit) {
     if (ld_bytes[i] != b.d[i])
       return false;
 
-  if (read_uninit && b.d[10]) // expected-note{{read of uninitialized object is not allowed in a constant expression}}
+  if (read_uninit && b.d[10]) // both-note{{read of uninitialized object is not allowed in a constant expression}}
     return false;
 
   return true;
 }
 
 static_assert(f(/*read_uninit=*/false), "");
-static_assert(f(/*read_uninit=*/true), ""); // expected-error{{static assertion expression is not an integral constant expression}} \
-                                            // expected-note{{in call to 'f(true)'}}
-#endif
+static_assert(f(/*read_uninit=*/true), ""); // both-error{{static assertion expression is not an integral constant expression}} \
+                                            // both-note{{in call to 'f(true)'}}
 constexpr bytes ld539 = {
   0x0, 0x0,  0x0,  0x0,
   0x0, 0x0,  0xc0, 0x86,