[clang] [clang][ARM] disable frame pointers by default for bare metal ARM targets (PR #117140)

[Ties Stuij via cfe-commits]

https://github.com/stuij created https://github.com/llvm/llvm-project/pull/117140

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 decrease code size

>From 4a85a0cd98bf328f31465d47c56640abdf7ec08c Mon Sep 17 00:00:00 2001
From: Ties Stuij <ties.stuij at arm.com>
Date: Fri, 15 Nov 2024 13:19:08 +0000
Subject: [PATCH] [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
---
 clang/lib/Driver/ToolChains/BareMetal.cpp  |  8 +++++-
 clang/lib/Driver/ToolChains/BareMetal.h    |  2 ++
 clang/lib/Driver/ToolChains/CommonArgs.cpp |  5 ++++
 clang/test/Driver/frame-pointer-elim.c     | 29 ++++++++++++++++++++++
 4 files changed, 43 insertions(+), 1 deletion(-)

diff --git a/clang/lib/Driver/ToolChains/BareMetal.cpp b/clang/lib/Driver/ToolChains/BareMetal.cpp
index f9a73f60973e4c..13b510e7e70994 100644
--- a/clang/lib/Driver/ToolChains/BareMetal.cpp
+++ b/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) {
diff --git a/clang/lib/Driver/ToolChains/BareMetal.h b/clang/lib/Driver/ToolChains/BareMetal.h
index b385c8cf76aab0..ae09bcedd78a28 100644
--- a/clang/lib/Driver/ToolChains/BareMetal.h
+++ b/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,
diff --git a/clang/lib/Driver/ToolChains/CommonArgs.cpp b/clang/lib/Driver/ToolChains/CommonArgs.cpp
index 8d977149e62485..8d54d0a8649cc9 100644
--- a/clang/lib/Driver/ToolChains/CommonArgs.cpp
+++ b/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;
 }
 
diff --git a/clang/test/Driver/frame-pointer-elim.c b/clang/test/Driver/frame-pointer-elim.c
index cdedcc7ae4c89f..667c47b34bc703 100644
--- a/clang/test/Driver/frame-pointer-elim.c
+++ b/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(); }