The runtime test whether the compiler supports arithmetic shift of negative signed numbers currently relies on undefined behavior in C, which means that all bets are off regarding whether the condition that follows passes or fails, regardless of whether the compiler in fact supports arithmetic shift or not.
Relevant quote from ISO C99 (6.5.7/4) The result of E1 << E2 is E1 left-shifted E2 bit positions; vacated bits are filled with zeros. If E1 has an unsigned type, the value of the result is E1 × 2^E2, reduced modulo one more than the maximum value representable in the result type. If E1 has a signed type and nonnegative value, and E1 × 2^E2 is representable in the result type, then that is the resulting value; otherwise, the behavior is undefined. For historic purposes, let's keep the test in place (although it is doubtful we actually need it) but rewrite it in a way that prevents compilers from this century from doing whacky things with it. Contributed-under: TianoCore Contribution Agreement 1.0 Signed-off-by: Ard Biesheuvel <[email protected]> --- Starting with version 3.7, Clang warns about the use of a negative left hand operand when performing arithmetic shift left, leading to build failures under -Werror. MdePkg/Library/BaseLib/Math64.c | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-) diff --git a/MdePkg/Library/BaseLib/Math64.c b/MdePkg/Library/BaseLib/Math64.c index 83d76847213e..9624cf90029f 100644 --- a/MdePkg/Library/BaseLib/Math64.c +++ b/MdePkg/Library/BaseLib/Math64.c @@ -86,7 +86,7 @@ InternalMathARShiftU64 ( // // Test if this compiler supports arithmetic shift // - TestValue = (((-1) << (sizeof (-1) * 8 - 1)) >> (sizeof (-1) * 8 - 1)); + TestValue = (INTN)((INT64)(1ULL << 63) >> 63); if (TestValue == -1) { // // Arithmetic shift is supported -- 2.5.0 _______________________________________________ edk2-devel mailing list [email protected] https://lists.01.org/mailman/listinfo/edk2-devel
