[Driver] Default to -momit-leaf-frame-pointer for AArch64

[MaskRay]

Summary

This matches https://gcc.gnu.org/onlinedocs/gcc/AArch64-Options.html

-momit-leaf-frame-pointer
-mno-omit-leaf-frame-pointer

Omit or keep the frame pointer in leaf functions. The former behavior is the default.

-mno-omit-leaf-frame-pointer is currently a no-op because
TargetOptions::DisableFramePointerElim is only considered for non-leaf
functions.

[MaskRay]

Ping:)

[sdesmalen]

The patch looks structurally fine, but I'm missing the argumentation for changing the default and related, the history of why the default is to not omit the frame pointer on leaf functions.
Can you provide some insight here?

[MaskRay]

This comment was removed by MaskRay.

[MaskRay]

(I got confused over the last weekend.)

@sdesmalen @efriedma We still need this driver change, otherwise D71168 will cause a functional difference when neither -m(no-)omit-leaf-frame-pointer is specified.

https://godbolt.org/z/653p3q

[efriedma]

If I'm understanding correctly, this does nothing without D71168? And together with D71168, the net change for clang is that "-mno-omit-leaf-frame-pointer" works correctly, but everything else stays the same?

[MaskRay]

In D71167#1784272, @efriedma wrote:

If I'm understanding correctly, this does nothing without D71168? And together with D71168, the net change for clang is that "-mno-omit-leaf-frame-pointer" works correctly, but everything else stays the same?

Yes.

[efriedma]

Okay, LGTM

clang/lib/Driver/ToolChains/Clang.cpp

getFramePointerKind(const ArgList &Args, const llvm::Triple &Triple) {
   //
   //  "omit" options taking precedence over "no-omit" options is the only way
   //  to make 3 valid states representable
   Arg *A = Args.getLastArg(options::OPT_fomit_frame_pointer,
                            options::OPT_fno_omit_frame_pointer);
   bool OmitFP = A && A->getOption().matches(options::OPT_fomit_frame_pointer);
   bool NoOmitFP =
       A && A->getOption().matches(options::OPT_fno_omit_frame_pointer);
-  bool KeepLeaf =
-      Args.hasFlag(options::OPT_momit_leaf_frame_pointer,
-                   options::OPT_mno_omit_leaf_frame_pointer, Triple.isPS4CPU());
+  bool KeepLeaf = Args.hasFlag(options::OPT_momit_leaf_frame_pointer,
+                               options::OPT_mno_omit_leaf_frame_pointer,
+                               Triple.isAArch64() || Triple.isPS4CPU());
   if (NoOmitFP || mustUseNonLeafFramePointerForTarget(Triple) ||
       (!OmitFP && useFramePointerForTargetByDefault(Args, Triple))) {
     if (KeepLeaf)
       return CodeGenOptions::FramePointerKind::NonLeaf;
     return CodeGenOptions::FramePointerKind::All;
   }
   return CodeGenOptions::FramePointerKind::None;
 }

clang/test/Driver/frame-pointer-elim.c

 // WARN-OMIT-7K: warning: optimization flag '-fomit-frame-pointer' is not supported for target 'armv7k'
 // WARN-OMIT-7K: "-mframe-pointer=all"
 
 // RUN: %clang -### -target armv7s-apple-ios8.0 -momit-leaf-frame-pointer %s 2>&1 | \
 // RUN:   FileCheck --check-prefix=WARN-OMIT-LEAF-7S %s
 // WARN-OMIT-LEAF-7S-NOT: warning: optimization flag '-momit-leaf-frame-pointer' is not supported for target 'armv7s'
 // WARN-OMIT-LEAF-7S: "-mframe-pointer=non-leaf"
 
-// On the PS4, we default to omitting the frame pointer on leaf functions
+// On AArch64 and PS4, default to omitting the frame pointer on leaf functions
+// RUN: %clang -### -target aarch64 -S %s 2>&1 | \
+// RUN:   FileCheck --check-prefix=KEEP-NON-LEAF %s
 // RUN: %clang -### -target x86_64-scei-ps4 -S %s 2>&1 | \
 // RUN:   FileCheck --check-prefix=KEEP-NON-LEAF %s
 // RUN: %clang -### -target x86_64-scei-ps4 -S -O2 %s 2>&1 | \
 // RUN:   FileCheck --check-prefix=KEEP-NON-LEAF %s
 
 // RUN: %clang -### -target powerpc64 -S %s 2>&1 | \
 // RUN:   FileCheck --check-prefix=KEEP-ALL %s
 // RUN: %clang -### -target powerpc64 -S -O1 %s 2>&1 | \
 // RUN:   FileCheck --check-prefix=KEEP-NONE %s
 
 void f0() {}
 void f1() { f0(); }