[PATCH] D60335: Use -fomit-frame-pointer when optimizing PowerPC code

[George Koehler via Phabricator via cfe-commits]

kernigh created this revision.
kernigh added reviewers: joerg, brad, cfe-commits.
Herald added subscribers: jsji, krytarowski, nemanjai.
Herald added a project: clang.

This enables -fomit-frame-pointer when optimizing code for all PowerPC
targets, instead of only Linux and NetBSD.

I mailed this patch to cfe-commits earlier this week.
Roman Lebedev and Hal Finkel pointed me to Phabricator;
this is my first attempt to use Phabricator.

My earlier mail wrote:

> The attached patch is for clang to use -fomit-frame-pointer by default
>  for all PowerPC targets when optimizing code.  Right now, clang uses
>  -fomit-frame-pointer for PowerPC Linux and NetBSD but not for other
>  targets.  I have been running `clang -target powerpc-openbsd`.
> 
> The patch is for llvm-project.git master.  I previously posted this
>  patch to https://bugs.llvm.org/show_bug.cgi?id=41094 , but the patch
>  in this email is for a newer revision of master.
> 
> In most functions, the frame pointer in r31 is an unnecessary extra
>  copy of the stack pointer in r1.  GCC is using -fomit-frame-pointer by
>  default (in my PowerPC machine running OpenBSD/macppc); I want Clang
>  to be at least as good as GCC.  Also, this patch helps me to compare
>  the output of `clang -target powerpc-openbsd -O2 -S` with the output
>  for Linux or NetBSD.  In bug 41094, I showed how -fomit-frame-pointer
>  simplifies the C function `void nothing(void) {}`.


Repository:
  rC Clang

https://reviews.llvm.org/D60335

Files:
  clang/lib/Driver/ToolChains/Clang.cpp


Index: clang/lib/Driver/ToolChains/Clang.cpp
===================================================================
--- clang/lib/Driver/ToolChains/Clang.cpp
+++ clang/lib/Driver/ToolChains/Clang.cpp
@@ -523,8 +523,12 @@
     // XCore never wants frame pointers, regardless of OS.
     // WebAssembly never wants frame pointers.
     return false;
+  case llvm::Triple::ppc:
+  case llvm::Triple::ppc64:
+  case llvm::Triple::ppc64le:
   case llvm::Triple::riscv32:
   case llvm::Triple::riscv64:
+    // PowerPC's frame pointer is often an extra copy of the stack pointer.
     return !areOptimizationsEnabled(Args);
   default:
     break;
@@ -542,9 +546,6 @@
     case llvm::Triple::mips64el:
     case llvm::Triple::mips:
     case llvm::Triple::mipsel:
-    case llvm::Triple::ppc:
-    case llvm::Triple::ppc64:
-    case llvm::Triple::ppc64le:
     case llvm::Triple::systemz:
     case llvm::Triple::x86:
     case llvm::Triple::x86_64:


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