[clang][ARM] disable frame pointers by default for bare metal ARM targets

because:
- This brings Clang in line with GCC for which this is the default for ARM
- It frees up a register, so performance increase, especially on Thumb/6-M
- It will also decrease code size

Author: stuij

clang/lib/Driver/ToolChains/BareMetal.cpp

@@ -128,8 +128,11 @@ BareMetal::BareMetal(const Driver &D, const llvm::Triple &Triple,
   }
 }
 
+namespace clang {
+namespace driver {
+namespace toolchains {
 /// Is the triple {arm,armeb,thumb,thumbeb}-none-none-{eabi,eabihf} ?
-static bool isARMBareMetal(const llvm::Triple &Triple) {
+bool isARMBareMetal(const llvm::Triple &Triple) {
   if (Triple.getArch() != llvm::Triple::arm &&
       Triple.getArch() != llvm::Triple::thumb &&
       Triple.getArch() != llvm::Triple::armeb &&
@@ -148,6 +151,9 @@ static bool isARMBareMetal(const llvm::Triple &Triple) {
 
   return true;
 }
+} // namespace clang
+} // namespace driver
+} // namespace clang
 
 /// Is the triple {aarch64.aarch64_be}-none-elf?
 static bool isAArch64BareMetal(const llvm::Triple &Triple) {

clang/lib/Driver/ToolChains/BareMetal.h

@@ -19,6 +19,8 @@ namespace driver {
 
 namespace toolchains {
 
+bool isARMBareMetal(const llvm::Triple &Triple);
+
 class LLVM_LIBRARY_VISIBILITY BareMetal : public ToolChain {
 public:
   BareMetal(const Driver &D, const llvm::Triple &Triple,

clang/lib/Driver/ToolChains/CommonArgs.cpp

@@ -19,6 +19,7 @@
 #include "Arch/SystemZ.h"
 #include "Arch/VE.h"
 #include "Arch/X86.h"
+#include "BareMetal.h"
 #include "HIPAMD.h"
 #include "Hexagon.h"
 #include "MSP430.h"
@@ -151,6 +152,10 @@ static bool useFramePointerForTargetByDefault(const llvm::opt::ArgList &Args,
     }
   }
 
+  if (toolchains::isARMBareMetal(Triple)) {
+    return false;
+  }
+
   return true;
 }
 

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

@@ -162,5 +162,34 @@
 // RUN:   FileCheck --check-prefix=KEEP-NON-LEAF %s
 // RUN: not %clang -### --target=riscv64-linux-android -mbig-endian -O1 -S %s 2>&1 | \
 // RUN:   FileCheck --check-prefix=KEEP-NON-LEAF %s
+
+// On ARM backend bare metal targets, frame pointer is omitted
+// RUN: %clang -### --target=arm-arm-none-eabi -S %s 2>&1 | \
+// RUN:   FileCheck --check-prefix=KEEP-NONE %s
+// RUN: %clang -### --target=arm-arm-none-eabihf -S %s 2>&1 | \
+// RUN:   FileCheck --check-prefix=KEEP-NONE %s
+// RUN: %clang -### --target=arm-arm-none-eabi -S -fno-omit-frame-pointer %s 2>&1 | \
+// RUN:   FileCheck --check-prefix=KEEP-ALL %s
+// RUN: %clang -### --target=arm-arm-none-eabihf -S -fno-omit-frame-pointer %s 2>&1 | \
+// RUN:   FileCheck --check-prefix=KEEP-ALL %s
+// RUN: %clang -### --target=arm-arm-none-eabi -S -O1 %s 2>&1 | \
+// RUN:   FileCheck --check-prefix=KEEP-NONE %s
+// RUN: %clang -### --target=arm-arm-none-eabihf -S -O1 %s 2>&1 | \
+// RUN:   FileCheck --check-prefix=KEEP-NONE %s
+// RUN: %clang -### --target=arm-arm-none-eabi -S -O1 -fno-omit-frame-pointer %s 2>&1 | \
+// RUN:   FileCheck --check-prefix=KEEP-ALL %s
+// RUN: %clang -### --target=arm-arm-none-eabihf -S -O1 -fno-omit-frame-pointer %s 2>&1 | \
+// RUN:   FileCheck --check-prefix=KEEP-ALL %s
+
+// AArch64 bare metal targets behave like hosted targets
+// RUN: %clang -### --target=aarch64-none-elf -S %s 2>&1 |  \
+// RUN:   FileCheck --check-prefix=KEEP-NON-LEAF %s
+// RUN: %clang -### --target=aarch64-none-elf -S -O1 %s 2>&1 |  \
+// RUN:   FileCheck --check-prefix=KEEP-NON-LEAF %s
+// RUN: %clang -### --target=aarch64-none-elf -S -fno-omit-frame-pointer %s 2>&1 |  \
+// RUN:   FileCheck --check-prefix=KEEP-NON-LEAF %s
+// RUN: %clang -### --target=aarch64-none-elf -S -O1 -fno-omit-frame-pointer %s 2>&1 |  \
+// RUN:   FileCheck --check-prefix=KEEP-NON-LEAF %s
+
 void f0() {}
 void f1() { f0(); }