-O3 Compiler Optimization Level Still Deemed Too Unsafe For The Linux Kernel

[indepe]

junk = *ptr;
if (ptr == NULL)
something();

After optimization, there is no call to something!
Dereferencing a null pointer is UB. The compiler is allowed to infer that ptr is not null. So the IF can be eliminated!

This would likely surprise any C coder who didn't already know it.

I think that GCC stopped exploiting this perfectly legal inference in kernel code because it surprised programmers too much.

Furthermore, In some kernel code, 0 is a valid address. So things are even more weird. The compiler doesn't know this. It is kind of paradoxical from the standpoint of the C language.

I think in this case the compiler should issue an error or at least a warning about the undefined behavior, instead of using it to optimize.

(Because the reverse assumption is just as valid: an attempt to dereference a pointer that is NULL in some cases. The code doesn't make any sense in the first place.)

In other words, it is a solvable problem.

[Hugh]

I think in this case the compiler should issue an error or at least a warning about the undefined behavior, instead of using it to optimize.

(Because the reverse assumption is just as valid: an attempt to dereference a pointer that is NULL in some cases. The code doesn't make any sense in the first place.)

In other words, it is a solvable problem.

This is a case where compiler culture is non-intuitive. And by the time you learn enough to understand it, you are likely absorbed into it and share it.

Explanation: if the programmer wrote *ptr, he is asserting that that is legal and thus that ptr is not NULL. The compiler, desperate for any advantage, uses/exploits this information.

I felt the same way as you when I first encountered optimizing compilers (in the late 1960s). I've written tools to use optimizing-like techniques to detect "anomalies" in programs. It's amazing how poorly programmers take nagging. I've been looking at diagnostics from Coverity Scan today and it is tiresome, if useful.

BTW, it is almost impossible to reply due to ads causing my window to jump around.

[indepe]

This is a case where compiler culture is non-intuitive. And by the time you learn enough to understand it, you are likely absorbed into it and share it.

Explanation: if the programmer wrote *ptr, he is asserting that that is legal and thus that ptr is not NULL. The compiler, desperate for any advantage, uses/exploits this information.

Yes, I understood. However when the compiler then encounters the "if (ptr == NULL)", it would (ideally) notice the contradiction, cancel the previous expectation, and issue a warning, if not an error. Instead of now using only half the information and insist on making an optimization based on a hint that has already been shown to be contradicted by a statement of more explicit nature.

So from my point of view, that is simply a compiler bug that should be fixed instead of given a pass. Even more so if that is "compiler culture". It kind of confirms Linus' complaint about compiler treatment of "undefined behavior". I can't see how I might "share" that "desperate" point of view. It would be a wrong priority.

I felt the same way as you when I first encountered optimizing compilers (in the late 1960s). I've written tools to use optimizing-like techniques to detect "anomalies" in programs. It's amazing how poorly programmers take nagging. I've been looking at diagnostics from Coverity Scan today and it is tiresome, if useful.

Maybe I should try out Coverity Scan. In any case, if I wrote code like the above, I certainly would want the compiler to tell me about it.

[perpetually high]

Forget -O3. Build the kernel with Clang and enable LTO. Simply pass LLVM=1 and LLVM_IAS=1 as arguments to make and you should be able to switch on LTO optimisation [/CODE]

It is so simple, even Michael should be able to do it.

Hey dude, I just wanted to say thanks, because I'm compiling the kernel as we speak with full LTO and I'm stoked. Been wanting to try it for a while.

I switched 2 weeks ago from -O3 -march=native to -O2 -march=native, and it's been great. So I'm excited to see if I can tell a different with clang and LTO.

Will chime back in afterwards if I notice a difference, but your hot tip was exactly what I needed to get going.

It took a little work for Ubuntu 20.04.2 understandably since not many people are doing this (lld was 10.0.0, needed 10.0.1, so I went balls out and got lld-12). I also needed libclang-12-dev, and then that was it.

Thanks again. Cheers

[sdack]

Hey dude, I just wanted to say thanks, because I'm compiling the kernel as we speak with full LTO and I'm stoked. ...

Check your kernel configuration and make sure only to compile those parts as modules that are not essential for you. The less gets compiled as a module the more benefits LTO will give you. Modules will get compiled with LTO, too, but it will only optimise the parts of a module that make the module, but without the full kernel scope. Hence compiling essential parts directly into the kernel allows for more inlining and better optimisations during the final LTO pass.

[perpetually high]

Check your kernel configuration and make sure only to compile those parts as modules that are not essential for you. The less gets compiled as a module the more benefits LTO will give you. Modules will get compiled with LTO, too, but it will only optimise the parts of a module that make the module, but without the full kernel scope. Hence compiling essential parts directly into the kernel allows for more inlining and better optimisations during the final LTO pass.

Yeah I got all of that stuff tweaked out to the max, appreciate the heads up though.

So it finished compiling, this was with make -j4 all:

LD [M] sound/usb/line6/snd-usb-variax.ko
LD [M] sound/usb/misc/snd-ua101.ko
LD [M] sound/usb/usx2y/snd-usb-us122l.ko
LD [M] sound/usb/usx2y/snd-usb-usx2y.ko
LD [M] sound/x86/snd-hdmi-lpe-audio.ko
LD [M] sound/xen/snd_xen_front.ko
linux-5.12.12 ❯ ls
arch crypto Documentation include kernel mm scripts tools COPYING Kconfig modules.builtin modules.order System.map vmlinux.symvers
block debian drivers init lib net security usr CREDITS MAINTAINERS modules.builtin.modinfo Module.symvers vmlinux
certs debian.master fs ipc LICENSES samples sound virt Kbuild Makefile modules-only.symvers README vmlinux.o

I'm not sure what to do now. Before I was using "fakeroot debian/rules binary-headers binary-generic binary-perarch" to build my kernel and that was giving me .deb's

I tried using "LLVM=1 LLVM_IAS=1 sudo make-kpkg -j 4 --rootcmd fakeroot --initrd --append-to-version=-lto kernel-image kernel-headers" but that uses GCC

Can you let me know what the next step is? Thanks again

EDIT: made some clarifications

[sdack]

So it finished compiling, this was with make -j4 all:

LD [M] sound/usb/line6/snd-usb-variax.ko
LD [M] sound/usb/misc/snd-ua101.ko
LD [M] sound/usb/usx2y/snd-usb-us122l.ko
LD [M] sound/usb/usx2y/snd-usb-usx2y.ko
LD [M] sound/x86/snd-hdmi-lpe-audio.ko
LD [M] sound/xen/snd_xen_front.ko
linux-5.12.12 ❯ ls
arch crypto Documentation include kernel mm scripts tools COPYING Kconfig modules.builtin modules.order System.map vmlinux.symvers
block debian drivers init lib net security usr CREDITS MAINTAINERS modules.builtin.modinfo Module.symvers vmlinux
certs debian.master fs ipc LICENSES samples sound virt Kbuild Makefile modules-only.symvers README vmlinux.o

I'm not sure what to do now. ...

The way I always do it is with a script. I have been doing this for decades now and a script serves me as a notepad but also to make it simple. The gist of it is this:

pushd /usr/src/linux
make LLVM=1 LLVM_IAS=1 distclean
cp ../latest-config .config     # copies the last good configuration over
make LLVM=1 LLVM_IAS=1 oldconfig prepare
make LLVM=1 LLVM_IAS=1 all
make LLVM=1 LLVM_IAS=1 install_modules     # for Arm do I also add a "install_dtbs" here
make LLVM=1 LLVM_IAS=1 install     # for Arm do I use "zinstall" as the target instead

pushd /usr/src/nvidia-xxx.yy     # compiling the Nvidia module next
export IGNORE_CC_MISMATCH=1
make LLVM=1 LLVM_IAS=1 clean
make LLVM=1 LLVM_IAS=1 KERNEL_UNAME="5.12.12" modules
make LLVM=1 LLVM_IAS=1 KERNEL_UNAME="5.12.12" modules_install

# whatever else you want to compile comes next, i.e. out-of-tree device drivers
# ...

popd     # Nvidia
popd     # linux kernel

update-initramfs -u -k 5.12.12     # update the initramdisk
update-grub     # update grub to include the new kernel

# On Arm do I also run "mkimage" to update the boot script so it boots the new kernel with dtb and initramdisk

That is the short version. Hope you can pick out what you need.

[perpetually high]

The way I always do it is with a script. I have been doing this for decades now and a script serves me as a notepad but also to make it simple. The gist of it is this:

pushd /usr/src/linux
make LLVM=1 LLVM_IAS=1 distclean
cp ../latest-config .config # copies the last good configuration over
make LLVM=1 LLVM_IAS=1 oldconfig prepare
make LLVM=1 LLVM_IAS=1 all
make LLVM=1 LLVM_IAS=1 install_modules # for Arm do I also add a "install_dtbs" here
make LLVM=1 LLVM_IAS=1 install # for Arm do I use "zinstall" as the target instead

pushd /usr/src/nvidia-xxx.yy # compiling the Nvidia module next
export IGNORE_CC_MISMATCH=1
make LLVM=1 LLVM_IAS=1 clean
make LLVM=1 LLVM_IAS=1 KERNEL_UNAME="5.12.12" modules
make LLVM=1 LLVM_IAS=1 KERNEL_UNAME="5.12.12" modules_install

# whatever else you want to compile comes next, i.e. out-of-tree device drivers
# ...

popd # Nvidia
popd # linux kernel

update-initramfs -u -k 5.12.12 # update the initramdisk
update-grub # update grub to include the new kernel

# On Arm do I also run "mkimage" to update the boot script so it boots the new kernel with dtb and initramdisk

That is the short version. Hope you can pick out what you need.

Thank you my good sir! Will give it a look and add to my script. Cheers : )

TIL about pushd, jesus. How'd I not know about this before