[Bitcode] Fix a signed integer overflow in BitstreamWriter.h

[MaggieYingYi]

UBSAN finds undefined behavior (signed integer overflow) in BitstreamWriter.h line 242 and 258.

To reproduce the issue, I have copied the functions EmitVBR and EmitVBR64 into a simple code main.cpp:

$ cat main.cpp
#include <cassert>
#include <cstdint>

void Emit(uint32_t Val, unsigned NumBits) {
  assert(NumBits && NumBits <= 32 && "Invalid value size!");
  assert((Val & ~(~0U >> (32-NumBits))) == 0 && "High bits set!");
}

void EmitVBR(uint32_t Val, unsigned NumBits) {
  assert(NumBits <= 32 && "Too many bits to emit!");
  uint32_t Threshold = 1U << (NumBits-1);

  // Emit the bits with VBR encoding, NumBits-1 bits at a time.
  while (Val >= Threshold) {
    Emit((Val & ((1 << (NumBits-1))-1)) | (1 << (NumBits-1)), NumBits);
    Val >>= NumBits-1;
  }

  Emit(Val, NumBits);
}

void EmitVBR64(uint64_t Val, unsigned NumBits) {
  assert(NumBits <= 32 && "Too many bits to emit!");
  if ((uint32_t)Val == Val)
    return EmitVBR((uint32_t)Val, NumBits);

  uint32_t Threshold = 1U << (NumBits-1);

  // Emit the bits with VBR encoding, NumBits-1 bits at a time.
  while (Val >= Threshold) {
    Emit(((uint32_t)Val & ((1 << (NumBits - 1)) - 1)) | (1 << (NumBits - 1)),
         NumBits);
    Val >>= NumBits-1;
  }

  Emit((uint32_t)Val, NumBits);
}

int main() {
  unsigned NumBits= 32;
  uint32_t UInt32 = 4294967295;
  uint64_t UInt64 = 1844674407370955161;
 
  EmitVBR64(UInt32, NumBits);
  EmitVBR64(UInt64, NumBits);
  return 0;
}

Compile the code using

To reproduce this issue, build main.cpp with the -fsanitize=undefined option:

clang++.exe -fsanitize=undefined main.cpp -o main.exe

Run the main.exe:

$ main.exe
main.cpp:15:36: runtime error: signed integer overflow: -2147483648 - 1 cannot be represented in type 'int'
SUMMARY: UndefinedBehaviorSanitizer: undefined-behavior main.cpp:15:36 in
main.cpp:31:49: runtime error: signed integer overflow: -2147483648 - 1 cannot be represented in type 'int'
SUMMARY: UndefinedBehaviorSanitizer: undefined-behavior main.cpp:31:49 in

The issue could be fixed by using 1U instead of 1.

[MaggieYingYi]

gentle pining ...

[nikic]

LGTM

[MaggieYingYi]

LGTM

Thanks.

[MaggieYingYi]

Committed