silvas added a subscriber: silvas. silvas added a comment. In http://reviews.llvm.org/D12271#231204, @rnk wrote:
> This looks good. > > As a larger issue, LLVM fast isel definitely won't pattern match this series > of shifts and selects to rotl at -O0. There are some users who want > branchless constant time rotates regardless of optimization level > (https://llvm.org/bugs/show_bug.cgi?id=24226). My thinking is that the shifts > are more analyzable to LLVM than an intrinsic, so we should leave these > intrinsics alone and tell such users to use inline asm if they need these > kinds of low-level guarantees. It still isn't very satisfactory. =/ I don't remember anything in PR24226 about "regardless of optimization level". In particular, code with "constant time" requirements won't tolerate -O0 codegen anyway because of the large amount of memory accesses into the stack -- the timing of such operations will depend on nasty things like cache line aliasing and such, which has similar problems to branches. http://reviews.llvm.org/D12271 _______________________________________________ cfe-commits mailing list cfe-commits@lists.llvm.org http://lists.llvm.org/cgi-bin/mailman/listinfo/cfe-commits
