RPCS3/llvm-mirror
1
0
Fork 0
mirror of https://github.com/RPCS3/llvm-mirror.git synced 2025-01-31 20:51:52 +01:00
Code Issues Projects Releases Wiki Activity

[Clang][A32/T32][Linux] -O1 implies -fomit-frame-pointer

Browse Source 
Summary:
An upgrade of LLVM for CrOS [0] containing [1] triggered a bunch of
errors related to writing to reserved registers for a Linux kernel's
arm64 compat vdso (which is a aarch32 image).

After a discussion on LKML [2], it was determined that
-f{no-}omit-frame-pointer was not being specified. Comparing GCC and
Clang [3], it becomes apparent that GCC defaults to omitting the frame
pointer implicitly when optimizations are enabled, and Clang does not.
ie. setting -O1 (or above) implies -fomit-frame-pointer. Clang was
defaulting to -fno-omit-frame-pointer implicitly unless -fomit-frame-pointer
was set explicitly.

Why this becomes a problem is that the Linux kernel's arm64 compat vdso
contains code that uses r7. r7 is used sometimes for the frame pointer
(for example, when targeting thumb (-mthumb)). See useR7AsFramePointer()
in llvm/llvm-project/llvm/lib/Target/ARM/ARMSubtarget.h. This is mostly
for legacy/compatibility reasons, and the 2019 Q4 revision of the ARM
AAPCS looks to standardize r11 as the frame pointer for aarch32, though
this is not yet implemented in LLVM.

Users that are reliant on the implicit value if unspecified when
optimizations are enabled should explicitly choose -fomit-frame-pointer
(new behavior) or -fno-omit-frame-pointer (old behavior).

[0] https://bugs.chromium.org/p/chromium/issues/detail?id=1084372
[1] https://reviews.llvm.org/D76848
[2] https://lore.kernel.org/lkml/20200526173117.155339-1-ndesaulniers@google.com/
[3] https://godbolt.org/z/0oY39t

Reviewers: kristof.beyls, psmith, danalbert, srhines, MaskRay, ostannard, efriedma

Reviewed By: psmith, danalbert, srhines, MaskRay, efriedma

Subscribers: efriedma, olista01, MaskRay, vhscampos, cfe-commits, llvm-commits, manojgupta, llozano, glider, hctim, eugenis, pcc, peter.smith, srhines

Tags: #clang, #llvm

Differential Revision: https://reviews.llvm.org/D80828
This commit is contained in:
Nick Desaulniers 2020-06-02 15:54:09 -07:00
parent 995cc9c405
commit 6cf6be73f3
1 changed files with 6 additions and 0 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

6
docs/ReleaseNotes.rst
View File

@ -91,6 +91,12 @@ During this release ...
* Implemented C-language intrinsics ``<arm_cde.h>`` for the CDE instruction set.
* Clang now defaults to ``-fomit-frame-pointer`` when targeting non-Android
Linux for arm and thumb when optimizations are enabled. Users that were
previously not specifying a value and relying on the implicit compiler
default may wish to specify ``-fno-omit-frame-pointer`` to get the old
behavior. This improves compatibility with GCC.
Changes to the MIPS Target
--------------------------