[Lldb-commits] [lldb] r353191 - [Obj-C] Fix undefined behaviour(s) in the new NSTaggedDate formatter.

[Davide Italiano via lldb-commits]

Shafik, Zachary, I think you reported this a bit ago, but now I got
around to fix it (also because it was breaking something real).

Thanks,

--
Davide

On Tue, Feb 5, 2019 at 9:30 AM Davide Italiano via lldb-commits
<lldb-commits at lists.llvm.org> wrote:
>
> Author: davide
> Date: Tue Feb  5 09:30:53 2019
> New Revision: 353191
>
> URL: http://llvm.org/viewvc/llvm-project?rev=353191&view=rev
> Log:
> [Obj-C] Fix undefined behaviour(s) in the new NSTaggedDate formatter.
>
> Type punning through a union -> no good.
> double to uint64 to double again -> no good either.
>
> The nice side effect, other than silencing the sanitizer bot
> is that it fixes the formatting of some dates, e.g. Jan 1st 1970.
>
> <rdar://problem/47617983>
>
> Modified:
>     lldb/trunk/source/Plugins/Language/ObjC/Cocoa.cpp
>
> Modified: lldb/trunk/source/Plugins/Language/ObjC/Cocoa.cpp
> URL: http://llvm.org/viewvc/llvm-project/lldb/trunk/source/Plugins/Language/ObjC/Cocoa.cpp?rev=353191&r1=353190&r2=353191&view=diff
> ==============================================================================
> --- lldb/trunk/source/Plugins/Language/ObjC/Cocoa.cpp (original)
> +++ lldb/trunk/source/Plugins/Language/ObjC/Cocoa.cpp Tue Feb  5 09:30:53 2019
> @@ -27,6 +27,7 @@
>  #include "lldb/Utility/Stream.h"
>
>  #include "llvm/ADT/APInt.h"
> +#include "llvm/ADT/bit.h"
>
>  #include "Plugins/LanguageRuntime/ObjC/AppleObjCRuntime/AppleObjCRuntime.h"
>
> @@ -749,24 +750,18 @@ bool lldb_private::formatters::NSURLSumm
>  ///   distantFuture, except within about 1e-25 second of the reference date.
>  const int TAGGED_DATE_EXPONENT_BIAS = 0x3ef;
>
> -typedef union {
> -  struct {
> -    uint64_t fraction:52;  // unsigned
> -    uint64_t exponent:11;  // signed
> -    uint64_t sign:1;
> -  } repr;
> -  uint64_t i;
> -  double d;
> -} DoubleBits;
> -typedef union {
> -  struct {
> -    uint64_t fraction:52;  // unsigned
> -    uint64_t exponent:7;   // signed
> -    uint64_t sign:1;
> -    uint64_t unused:4;  // placeholder for pointer tag bits
> -  } repr;
> -  uint64_t i;
> -} TaggedDoubleBits;
> +struct DoubleBits {
> +  uint64_t fraction : 52; // unsigned
> +  uint64_t exponent : 11; // signed
> +  uint64_t sign : 1;
> +};
> +
> +struct TaggedDoubleBits {
> +  uint64_t fraction : 52; // unsigned
> +  uint64_t exponent : 7;  // signed
> +  uint64_t sign : 1;
> +  uint64_t unused : 4; // placeholder for pointer tag bits
> +};
>
>  static uint64_t decodeExponent(uint64_t exp) {
>    // Tagged exponent field is 7-bit signed. Sign-extend the value to 64 bits
> @@ -774,24 +769,24 @@ static uint64_t decodeExponent(uint64_t
>    return llvm::SignExtend64<7>(exp) + TAGGED_DATE_EXPONENT_BIAS;
>  }
>
> -static uint64_t decodeTaggedTimeInterval(uint64_t encodedTimeInterval) {
> +static double decodeTaggedTimeInterval(uint64_t encodedTimeInterval) {
>    if (encodedTimeInterval == 0)
>      return 0.0;
>    if (encodedTimeInterval == std::numeric_limits<uint64_t>::max())
>      return (uint64_t)-0.0;
>
> -  TaggedDoubleBits encodedBits = {};
> -  encodedBits.i = encodedTimeInterval;
> -  DoubleBits decodedBits;
> +  TaggedDoubleBits encodedBits =
> +      llvm::bit_cast<TaggedDoubleBits>(encodedTimeInterval);
> +  assert(encodedBits.unused == 0);
>
>    // Sign and fraction are represented exactly.
>    // Exponent is encoded.
> -  assert(encodedBits.repr.unused == 0);
> -  decodedBits.repr.sign = encodedBits.repr.sign;
> -  decodedBits.repr.fraction = encodedBits.repr.fraction;
> -  decodedBits.repr.exponent = decodeExponent(encodedBits.repr.exponent);
> +  DoubleBits decodedBits;
> +  decodedBits.sign = encodedBits.sign;
> +  decodedBits.fraction = encodedBits.fraction;
> +  decodedBits.exponent = decodeExponent(encodedBits.exponent);
>
> -  return decodedBits.d;
> +  return llvm::bit_cast<double>(decodedBits);
>  }
>
>  bool lldb_private::formatters::NSDateSummaryProvider(
> @@ -868,7 +863,8 @@ bool lldb_private::formatters::NSDateSum
>
>    // Accomodate for the __NSTaggedDate format introduced in Foundation 1600.
>    if (class_name == g___NSTaggedDate) {
> -    auto *runtime = llvm::dyn_cast_or_null<AppleObjCRuntime>(process_sp->GetObjCLanguageRuntime());
> +    auto *runtime = llvm::dyn_cast_or_null<AppleObjCRuntime>(
> +        process_sp->GetObjCLanguageRuntime());
>      if (runtime && runtime->GetFoundationVersion() >= 1600)
>        date_value = decodeTaggedTimeInterval(value_bits << 4);
>    }
>
>
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