[PATCH] D80828: [Clang][A32/T32][Linux] -O2 implies -fomit-frame-pointer

[Nick Desaulniers via Phabricator via cfe-commits]

nickdesaulniers created this revision.
nickdesaulniers added reviewers: kristof.beyls, psmith, olista01.
Herald added projects: clang, LLVM.
Herald added subscribers: llvm-commits, cfe-commits.
nickdesaulniers added a subscriber: vhscampos.

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


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D80828

Files:
  clang/lib/Driver/ToolChains/Clang.cpp
  clang/test/Driver/frame-pointer-elim.c
  llvm/docs/ReleaseNotes.rst


Index: llvm/docs/ReleaseNotes.rst
===================================================================
--- llvm/docs/ReleaseNotes.rst
+++ llvm/docs/ReleaseNotes.rst
@@ -91,6 +91,12 @@
 
 * Implemented C-language intrinsics ``<arm_cde.h>`` for the CDE instruction set.
 
+* Clang now defaults to `-fomit-frame-pointer` when targeting 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
 --------------------------
 
Index: clang/test/Driver/frame-pointer-elim.c
===================================================================
--- clang/test/Driver/frame-pointer-elim.c
+++ clang/test/Driver/frame-pointer-elim.c
@@ -103,5 +103,24 @@
 // RUN: %clang -### -target powerpc64 -S -O1 %s 2>&1 | \
 // RUN:   FileCheck --check-prefix=KEEP-NONE %s
 
+// For AAarch32 (A32, T32) linux targets, default omit frame pointer when
+// optimizations are enabled.
+// RUN: %clang -### -target arm-linux-gnueabihf- -marm -S %s 2>&1 | \
+// RUN:   FileCheck --check-prefix=KEEP-ALL %s
+// RUN: %clang -### -target arm-linux-gnueabihf- -mthumb -S %s 2>&1 | \
+// RUN:   FileCheck --check-prefix=KEEP-ALL %s
+// RUN: %clang -### -target arm-linux-gnueabihf- -marm -mbig-endian -S %s 2>&1 | \
+// RUN:   FileCheck --check-prefix=KEEP-ALL %s
+// RUN: %clang -### -target arm-linux-gnueabihf- -mthumb -mbig-endian -S %s 2>&1 | \
+// RUN:   FileCheck --check-prefix=KEEP-ALL %s
+// RUN: %clang -### -target arm-linux-gnueabihf- -marm -O2 -S %s 2>&1 | \
+// RUN:   FileCheck --check-prefix=KEEP-NONE %s
+// RUN: %clang -### -target arm-linux-gnueabihf- -mthumb -O2 -S %s 2>&1 | \
+// RUN:   FileCheck --check-prefix=KEEP-NONE %s
+// RUN: %clang -### -target arm-linux-gnueabihf- -marm -mbig-endian -O2 -S %s 2>&1 | \
+// RUN:   FileCheck --check-prefix=KEEP-NONE %s
+// RUN: %clang -### -target arm-linux-gnueabihf- -mthumb -mbig-endian -O2 -S %s 2>&1 | \
+// RUN:   FileCheck --check-prefix=KEEP-NONE %s
+
 void f0() {}
 void f1() { f0(); }
Index: clang/lib/Driver/ToolChains/Clang.cpp
===================================================================
--- clang/lib/Driver/ToolChains/Clang.cpp
+++ clang/lib/Driver/ToolChains/Clang.cpp
@@ -557,11 +557,15 @@
       Triple.isOSHurd()) {
     switch (Triple.getArch()) {
     // Don't use a frame pointer on linux if optimizing for certain targets.
+    case llvm::Triple::arm:
+    case llvm::Triple::armeb:
     case llvm::Triple::mips64:
     case llvm::Triple::mips64el:
     case llvm::Triple::mips:
     case llvm::Triple::mipsel:
     case llvm::Triple::systemz:
+    case llvm::Triple::thumb:
+    case llvm::Triple::thumbeb:
     case llvm::Triple::x86:
     case llvm::Triple::x86_64:
       return !areOptimizationsEnabled(Args);


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