[SPARC][clang] Enable frame pointer optimization by default

[koakuma]

Summary

Enable frame pointer optimization by default to match it with other targets.
This brings a small reduction in generated binary sizes.

Fixes bug #48327 (https://github.com/llvm/llvm-project/issues/48327).

[MaskRay]

Enable frame pointer optimization by default to match it with other targets.

I think we usually follow GCC instead of other targets.

I am unfamiliar with sparc. Can you show an example where -fomit-frame-pointer/-fno-omit-frame-pointer demonstrate a codegen difference? I tried some simple examples and cannot reproduce a difference.

It's somewhat unfortunate to add 6 RUN lines to test these sparc behaviors...

[koakuma]

In D138532#3945422, @MaskRay wrote:

I think we usually follow GCC instead of other targets.

GCC enables it by default too, for example see https://godbolt.org/z/brPMKnM17
(That link also has comparison with clang with -fomit-frame-pointer/-fno-omit-frame-pointer)

I am unfamiliar with sparc. Can you show an example where -fomit-frame-pointer/-fno-omit-frame-pointer demonstrate a codegen difference? I tried some simple examples and cannot reproduce a difference.

Unfortunately on SPARC, due to the register window mechanics this optimization can only be done on leaf functions.
As for the concrete effects, it skips the save and restore instructions for register window management, which, among other things, does the saving/restoring of frame pointers, and changes the return instruction to retl (aka jmp %o7+8).

[arsenm]

LGTM. I still find it strange that by default clang explicitly disables an optimization that's the default for unattributed functions

clang/lib/Driver/ToolChains/Clang.cpp

case llvm::Triple::msp430:
     // WebAssembly never wants frame pointers.
     return false;
   case llvm::Triple::ppc:
   case llvm::Triple::ppcle:
   case llvm::Triple::ppc64:
   case llvm::Triple::ppc64le:
   case llvm::Triple::riscv32:
   case llvm::Triple::riscv64:
+  case llvm::Triple::sparc:
+  case llvm::Triple::sparcel:
+  case llvm::Triple::sparcv9:
   case llvm::Triple::amdgcn:
   case llvm::Triple::r600:
   case llvm::Triple::csky:
   case llvm::Triple::loongarch32:
   case llvm::Triple::loongarch64:
     return !areOptimizationsEnabled(Args);
   default:
     break;

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

 // RUN: %clang -### -target ve-unknown-linux-gnu -S %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
 
+// SPARC targets omit the frame pointer when optimizations are enabled.
+// RUN: %clang -### -target sparc -S %s 2>&1 | \
+// RUN:   FileCheck --check-prefix=KEEP-ALL %s
+// RUN: %clang -### -target sparc -S -O1 %s 2>&1 | \
+// RUN:   FileCheck --check-prefix=KEEP-NONE %s
+// RUN: %clang -### -target sparcel -S %s 2>&1 | \
+// RUN:   FileCheck --check-prefix=KEEP-ALL %s
+// RUN: %clang -### -target sparcel -S -O1 %s 2>&1 | \
+// RUN:   FileCheck --check-prefix=KEEP-NONE %s
+// RUN: %clang -### -target sparc64 -S %s 2>&1 | \
+// RUN:   FileCheck --check-prefix=KEEP-ALL %s
+// RUN: %clang -### -target sparc64 -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