[libc][test] -Werror=maybe-uninitialized in high_precision_decimal_test.cpp

[nickdesaulniers]

The unit tests fail with gcc-14:

In file included from /android0/llvm-project/libc/test/src/__support/high_precision_decimal_test.cpp:9:
In member function ‘bool __llvm_libc_20_0_0_git::internal::HighPrecisionDecimal::should_round_up(int32_t, __llvm_libc_20_0_0_git::internal::RoundDirection)’,
    inlined from ‘T __llvm_libc_20_0_0_git::internal::HighPrecisionDecimal::round_to_integer_type(__llvm_libc_20_0_0_git::internal::RoundDirection) [with T = long unsigned int]’ at /android0/llvm-project/libc/src/__support/high_precision_decimal.h:428:46,
    inlined from ‘virtual void LlvmLibcHighPrecisionDecimalTest_NumLenDigitsTest::Run()’ at /android0/llvm-project/libc/test/src/__support/high_precision_decimal_test.cpp:435:3:
/android0/llvm-project/libc/src/__support/high_precision_decimal.h:150:36: error: ‘longer_hpd’ may be used uninitialized [-Werror=maybe-uninitialized]
  150 |     if (this->digits[round_to_digit] == 5 &&
      |         ~~~~~~~~~~~~~~~~~~~~~~~~~~~^
/android0/llvm-project/libc/test/src/__support/high_precision_decimal_test.cpp: In member function ‘virtual void LlvmLibcHighPrecisionDecimalTest_NumLenDigitsTest::Run()’:
/android0/llvm-project/libc/test/src/__support/high_precision_decimal_test.cpp:431:50: note: ‘longer_hpd’ declared here
  431 |   LIBC_NAMESPACE::internal::HighPrecisionDecimal longer_hpd =
      |                                                  ^~~~~~~~~~

In this case, I think GCC is correct. The constructor for HighPrecisionDecimal doesn't guarantee that its digits member is fully initialized, so constructing an object then calling round_to_integer_type may end up reading uninitialized memory (UB). I think the constructor of HighPrecisionDecimal should unconditionally initialize digits. Found while trying to test #124036.

[llvmbot]

@llvm/issue-subscribers-libc

Author: Nick Desaulniers (nickdesaulniers)

The unit tests fail with gcc-14: ``` In file included from /android0/llvm-project/libc/test/src/__support/high_precision_decimal_test.cpp:9: In member function ‘bool __llvm_libc_20_0_0_git::internal::HighPrecisionDecimal::should_round_up(int32_t, __llvm_libc_20_0_0_git::internal::RoundDirection)’, inlined from ‘T __llvm_libc_20_0_0_git::internal::HighPrecisionDecimal::round_to_integer_type(__llvm_libc_20_0_0_git::internal::RoundDirection) [with T = long unsigned int]’ at /android0/llvm-project/libc/src/__support/high_precision_decimal.h:428:46, inlined from ‘virtual void LlvmLibcHighPrecisionDecimalTest_NumLenDigitsTest::Run()’ at /android0/llvm-project/libc/test/src/__support/high_precision_decimal_test.cpp:435:3: /android0/llvm-project/libc/src/__support/high_precision_decimal.h:150:36: error: ‘longer_hpd’ may be used uninitialized [-Werror=maybe-uninitialized] 150 | if (this->digits[round_to_digit] == 5 && | ~~~~~~~~~~~~~~~~~~~~~~~~~~~^ /android0/llvm-project/libc/test/src/__support/high_precision_decimal_test.cpp: In member function ‘virtual void LlvmLibcHighPrecisionDecimalTest_NumLenDigitsTest::Run()’: /android0/llvm-project/libc/test/src/__support/high_precision_decimal_test.cpp:431:50: note: ‘longer_hpd’ declared here 431 | LIBC_NAMESPACE::internal::HighPrecisionDecimal longer_hpd = | ^~~~~~~~~~ ``` In this case, I think GCC is correct. The constructor for `HighPrecisionDecimal` doesn't guarantee that its `digits` member is fully initialized, so constructing an object then calling `round_to_integer_type` may end up reading uninitialized memory (UB). I think the constructor of `HighPrecisionDecimal` should unconditionally initialize `digits`.