Jeff Law <l...@redhat.com> writes: > On 08/12/2015 12:32 PM, Richard Biener wrote: >> On August 12, 2015 8:07:13 PM GMT+02:00, Jeff Law <l...@redhat.com> wrote: >>> On 08/12/2015 11:12 AM, Richard Biener wrote: >>> >>>> >>>> Prec is almost never a constant and is heavily used from wide-int. >>>> >>>> We are not exploiting this undefined ness in C so I object to making >>> this so much slower. >>>> >>>> Can we instead do what we do for abs_hwi and add a checking assert so >>> we can move the tests to the callers that misbehave instead? >>> Given that ISO C++ is moving away from making shifting 1 into the sign >>> bit undefined behaviour, maybe we should make UBSan less strict in its >>> warning. That may eliminate the need for Mikael's patch. >> >> We can also use an logical left shift followed by an arithmetic right >> shift. Or is the latter invoking undefined behaviour as well in some >> cases we hit? > Hmm, why aren't we using logicals for the left shift to begin with? > That's the problem area. I don't think the right shifts are an issue at > all.
Well, they're implementation-defined, at least in C. The C11 wording for E1 >> E2 is "If E1 has a signed type and a negative value, the resulting value is implementation-defined". Is C++ different? (I don't have the standard handy.) So... > It's strange that when I was researching this, consistently I saw folks > suggesting the LEFT-SHIFT followed by RIGHT-SHIFT, then it'd get shot > down as UB, then folks went to either Mikael's approach or another that > is very similar. Nobody suggested twidding the types to get a > left-logical-shift, then doing a right-arithmetic-shift. ...unless C++ is different, there's not a standard-level concept of arithmetic shift. Thanks, Richard
