On 2019-02-19 14:50:13, Brian J. Johnson wrote: > On 2/18/19 3:32 AM, Ard Biesheuvel wrote: > > On Mon, 18 Feb 2019 at 10:08, Jordan Justen <jordan.l.jus...@intel.com> > > wrote: > >> > >> On 2019-02-17 23:53:01, Ard Biesheuvel wrote: > >>> On Mon, 18 Feb 2019 at 05:12, Jordan Justen <jordan.l.jus...@intel.com> > >>> wrote: > >>>> > >>> > >>> This needs an explanation why optimization needs to be disabled. > >> > >> I'm not sure this is required. The reason I added these patches is to > >> hopefully prevent the compiler from removing the frame pointer. We > >> adjust the frame pointer in the code, and that is a little sketchy if > >> the frame pointer isn't being used. > >> > >> Unfortunately, it can reasonably be argued that the > >> TemporaryRamSupport PPI definition ultimately makes it unsafe to write > >> the migration code in C. > >> > >> I tried reverting both the EmulatorPkg and OvmfPkg patches for > >> disabling the optimizations, and with my setup there was no impact. I > >> think there is a good change that we'd be pretty safe to just drop > >> these two patches to wait and see if someone encounters a situation > >> that requires it. > >> > >> Ok, so based on this explanation, do you think I should add info to > >> the commit message and keep the patches, or just drop them? > >> > > > > I think 'little sketchy' is an understatement here (as is > > setjmp/longjmp in general), but it is the reality we have to deal with > > when writing startup code in C. Looking at the code, I agree that the > > fact that [re]bp is assigned directly implies that we should not > > permit it to be used as a general purpose register, especially when > > you throw LTO into the mix, which could produce all kinds of > > surprising results when it decides to inline functions being called > > from here. > > > > For GCC/Clang, I don't think it is correct to assume that changing the > > optimization level will result in -fno-omit-frame-pointer to be set, > > so I'd prefer setting that option directly, either via the pragma, or > > for the whole file. > > > > For MSVC, I have no idea how to tweak the compiler to force it to emit > > frame pointers. > > > > https://docs.microsoft.com/en-us/cpp/build/reference/oy-frame-pointer-omission?view=vs-2017
Hmm, and based on: https://docs.microsoft.com/en-us/cpp/preprocessor/optimize?view=vs-2017 #pragma optimize ("", off) ...includes the "y" option. This 2nd page seems a little confused, as it documents "y" as "Generate frame pointers on the program stack", while the 1st page says "Suppresses creation of frame pointers on the call stack". I think the "suppress" is more accurate as it makes more sense that suppressing the frame pointer gives better optimization opportunities. Anyway, I think that means that `#pragma optimize ("", off)` does what we want on MSVC to force frame pointers to be used. -Jordan _______________________________________________ edk2-devel mailing list edk2-devel@lists.01.org https://lists.01.org/mailman/listinfo/edk2-devel
