From 0c33a58da02148a104bc01d3fdb44be45f03fbe8 Mon Sep 17 00:00:00 2001
From: olemorud <ole.kristian.morud@cern.ch>
Date: Tue, 6 Jun 2023 11:32:12 +0200
Subject: [PATCH] Test pseudo-namespaces in C

In C you can create pseudo-namespaces by abusing structs of function
pointers, making function calls look like `module.func(x)` instead of
`func(x)`.

A lot of people online spout that this creates overhead, but this should
show that clang>=15.0.7 will still inline such function calls. GCC 11.3.1
is unable  to inline the function, but will still call the function
directly.
---
 CMakeLists.txt | 20 ++++++++++++++++++++
 Makefile       | 29 +++++++++++++++++++++++++++++
 build.sh       | 21 +++++++++++++++++++++
 dump.asm       | 10 ++++++++++
 main.c         | 22 ++++++++++++++++++++++
 module.c       | 12 ++++++++++++
 module.h       |  8 ++++++++
 7 files changed, 122 insertions(+)
 create mode 100644 CMakeLists.txt
 create mode 100644 Makefile
 create mode 100755 build.sh
 create mode 100644 dump.asm
 create mode 100644 main.c
 create mode 100644 module.c
 create mode 100644 module.h

diff --git a/CMakeLists.txt b/CMakeLists.txt
new file mode 100644
index 0000000..5f75245
--- /dev/null
+++ b/CMakeLists.txt
@@ -0,0 +1,20 @@
+
+cmake_minimum_required(VERSION 3.13)
+
+project(Namespaces)
+
+add_executable(main
+    module.c
+    main.c)
+
+target_compile_options(main
+	PRIVATE
+	-Werror -Wall -Wextra -O3 -g -std=c2x -flto)
+
+target_link_options(main
+	PRIVATE
+	-flto -Wl,-O2)
+
+target_compile_definitions(main
+	PRIVATE
+	-DNAMESPACES)
diff --git a/Makefile b/Makefile
new file mode 100644
index 0000000..e0addf7
--- /dev/null
+++ b/Makefile
@@ -0,0 +1,29 @@
+
+CC ?= clang
+
+# compiler specific flags
+CFLAGS.gcc := 
+CFLAGS.clang := 
+
+# general flags
+CFLAGS := \
+	-Wall -Wextra -Werror -Wpedantic \
+	-O3 -g -flto \
+	-std=c2x \
+	${CFLAGS.${CC}} \
+	-DNAMESPACES \
+
+LDFLAGS := \
+	-flto \
+	-Wl,-O3
+
+dump.asm : test
+	gdb $< -batch -ex 'disassemble main' > $@
+	(gdb $< -batch -ex 'disassemble succ' >> $@) || true
+
+test : main.o module.o
+	$(CC) $(CFLAGS) -o $@ $^
+
+%.o : %.c
+	$(CC) $(CFLAGS) -o $@ -c $^
+
diff --git a/build.sh b/build.sh
new file mode 100755
index 0000000..7017255
--- /dev/null
+++ b/build.sh
@@ -0,0 +1,21 @@
+#!/bin/bash
+
+requirements=("cmake" "gdb")
+
+for cmd in $requirements; do
+	if ! command -v gdb; then
+		echo "gdb not found"
+		exit 1
+	fi
+done
+
+mkdir -p build 
+(cd build
+	rm * -rf 
+	CC=clang cmake ..
+	make -j2
+)
+
+gdb ./build/main -batch -ex 'disassemble main' > dump.asm
+gdb ./build/main -batch -ex 'disassemble succ' 1>> dump.asm 2>/dev/null || true
+
diff --git a/dump.asm b/dump.asm
new file mode 100644
index 0000000..7b43e0a
--- /dev/null
+++ b/dump.asm
@@ -0,0 +1,10 @@
+Dump of assembler code for function main:
+   0x0000000000401130 <+0>:	push   %rax
+   0x0000000000401131 <+1>:	mov    $0x402010,%edi
+   0x0000000000401136 <+6>:	mov    $0x6,%esi
+   0x000000000040113b <+11>:	xor    %eax,%eax
+   0x000000000040113d <+13>:	call   0x401030 <printf@plt>
+   0x0000000000401142 <+18>:	xor    %eax,%eax
+   0x0000000000401144 <+20>:	pop    %rcx
+   0x0000000000401145 <+21>:	ret    
+End of assembler dump.
diff --git a/main.c b/main.c
new file mode 100644
index 0000000..53603a4
--- /dev/null
+++ b/main.c
@@ -0,0 +1,22 @@
+
+#include <stdio.h>
+
+#include "module.h"
+
+int main()
+{
+	int x = 4;
+
+#ifdef NAMESPACES
+#pragma message "namespaces enabled"
+	x = module.succ(x);
+	x = module.succ(x);
+#else
+	x = succ(x);
+	x = succ(x);
+#endif
+
+	printf("%d\n", x);
+
+	return 0;
+}
diff --git a/module.c b/module.c
new file mode 100644
index 0000000..a66f545
--- /dev/null
+++ b/module.c
@@ -0,0 +1,12 @@
+
+#include "module.h"
+
+inline int succ(int x)
+{
+	return x+1;
+}
+
+struct exports module = {
+	.succ = succ,
+};
+
diff --git a/module.h b/module.h
new file mode 100644
index 0000000..0acef84
--- /dev/null
+++ b/module.h
@@ -0,0 +1,8 @@
+
+int succ(int x);
+
+struct exports {
+	int (*succ)(int);
+};
+
+extern struct exports module;