On Fri, Mar 12, 2021 at 9:55 AM Nick Desaulniers <ndesaulniers@xxxxxxxxxx> wrote: > > On Thu, Mar 11, 2021 at 5:09 PM Nick Desaulniers > <ndesaulniers@xxxxxxxxxx> wrote: > > > > -Wframe-larger-than= requires stack frame information, which the > > frontend cannot provide. This diagnostic is emitted late during > > compilation once stack frame size is available. > > > > When building with LTO, the frontend simply lowers C to LLVM IR and does > > not have stack frame information, so it cannot emit this diagnostic. > > When the linker drives LTO, it restarts optimizations and lowers LLVM IR > > to object code. At that point, it has stack frame information but > > doesn't know to check for a specific max stack frame size. > > > > I consider this a bug in LLVM that we need to fix. There are some > > details we're working out related to LTO such as which value to use when > > there are multiple different values specified per TU, or how to > > propagate these to compiler synthesized routines properly, if at all. > > > > Until it's fixed, ensure we don't miss these. At that point we can wrap > > this in a compiler version guard or revert this based on the minimum > > support version of Clang. > > > > The error message is not generated during link: > > LTO vmlinux.o > > ld.lld: warning: stack size limit exceeded (8224) in foobarbaz > > > > Cc: Sami Tolvanen <samitolvanen@xxxxxxxxxx> > > Reported-by: Candle Sun <candlesea@xxxxxxxxx> > > Suggested-by: Fangrui Song <maskray@xxxxxxxxxx> > > Signed-off-by: Nick Desaulniers <ndesaulniers@xxxxxxxxxx> > > --- > > LTO users might want to `make clean` or `rm -rf .thinlto-cache` to test > > this. > > > > Makefile | 5 +++++ > > 1 file changed, 5 insertions(+) > > > > diff --git a/Makefile b/Makefile > > index f9b54da2fca0..74566b1417b8 100644 > > --- a/Makefile > > +++ b/Makefile > > Candle sent me a private message that we probably also want coverage > for kernel modules. Let me revise this and test/send a v2. False alarm, seems specific to Android's LTO support pre-5.11. I will fix that in Android trees. This patch is still relevant going forward. > > > @@ -910,6 +910,11 @@ CC_FLAGS_LTO += -fvisibility=hidden > > > > # Limit inlining across translation units to reduce binary size > > KBUILD_LDFLAGS += -mllvm -import-instr-limit=5 > > + > > +# Check for frame size exceeding threshold during prolog/epilog insertion. > > +ifneq ($(CONFIG_FRAME_WARN),0) > > +KBUILD_LDFLAGS += -plugin-opt=-warn-stack-size=$(CONFIG_FRAME_WARN) > > +endif > > endif > > > > ifdef CONFIG_LTO > > -- > > 2.31.0.rc2.261.g7f71774620-goog > > > > > -- > Thanks, > ~Nick Desaulniers -- Thanks, ~Nick Desaulniers
