missing mflr/mtlr (link register save/restore) when using inline asm w/ "lr" clobber

[nickdesaulniers]

Bugzilla Link	50147
Resolution	FIXED
Resolved on	Jun 03, 2021 23:16
Version	trunk
OS	All
Blocks	#4440 #48661
CC	@nathanchance,@nemanjai,@stephenhines,@stefanp-ibm,@tstellar,@uweigand
Fixed by commit(s)	15051f0 f2ce10d

Extended Description

example:
void foo (void) {
asm ("":::"lr");
}

powerpc64le-linux-gnu-gcc -O2:
foo:
mflr 0
std 0,16(1)
ld 0,16(1)
mtlr 0
blr

clang --target=powerpc64le-linux-gnu -O2:
foo:
blr

This is causing ppc32 Linux kernels to hang when powering down, resulting in CI failures, as reported by Nathan Chancellor and debugged by Christophe Leroy in https://lists.ozlabs.org/pipermail/linuxppc-dev/2021-April/228246.html.

[nickdesaulniers]

assigned to @stefanp-ibm

[nemanjai]

Yes, we are aware of this unfortunate omission in the PPC back end. In fact, this same issue exists for other special registers (R2, R1, etc.)

[stefanp-ibm]

Put up an initial patch on Phabricator:
https://reviews.llvm.org/D101657

The patch should fix the issue where the LR is clobbered in inline ASM.

[stefanp-ibm]

Should probably backport this to 12.0.1.
Marked this bug as blocking that release.

[nathanchance]

https://reviews.llvm.org/rG15051f0b4a2e0a0af9da7cd5e5cfaabb9f6aaa3d

Tom, could you please make sure this gets applied to release/12.x? I have verified that the issue that was initially reported is resolved and all the tests pass.

[tstellar]

Merged: f2ce10d