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. > @@ -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
