> On 12 May 2020, at 11:41, Andreas Schultz <andreas.schu...@travelping.com> > wrote: > > Hi, > > There are few places in VPP (most notable when trying to convert a netmask > into a length) that can pass 0 (zero) into min_log2 and expect to get a > meaningful result. > > Obviously log2(0) is undefined. It turns out that this also applies to the > return of min_log2. Under the hood the function uses __builtin_clzl, and the > return of that function is also undefined for input 0. > > __builtin_clzl could be replaced with __builtin_ia32_lzcnt_u64 on supported > CPUs to avoid the undefined behaviour. That would still not fix the problem > that passing 0 into a log function is broken by design.
Compiler uses LZCNT for __builtin_clzl when target -march supports it. Today we use -march=corei7 as baseline and that one doesn’t know about LZCNT, so BSR is emitted instead, which indeed have undefined behaviour. In other words replacing __builtin_clzl with __builtin_ia32_lzcnt_u64 will not work, but simply changing baseline -march= to something newer will address that transparently...
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