Use opt-level=z to shave off eight bytes

rustc/LLVM does constant propagation through the inline assembly
and finds more compact instructions!

Author: tormol

README.md

@@ -1,10 +1,11 @@
-# Using Rust to make a 145-byte static AMD64 Linux binary
+# Using Rust to make a 137-byte static AMD64 Linux binary
 
 Requires nightly Rust because it uses the `sc` crate to make direct
 system calls.
 
 `elf.s` contains a custom ELF header, but no instructions.
-All of the machine code comes out of `rustc`. (Although all except one of the instructions that survive optimization *went in* as inline assembly.)
+All of the machine code comes out of `rustc`.
+(While most of the operations originate from inline assembly, LLVM replaces the instructions with more compact ones!)
 
 ```
 $ ./build.sh
@@ -14,9 +15,9 @@ You have  rustc 1.46.0-nightly (346aec9b0 2020-07-11)
 Cloning into 'syscall.rs'...
    Compiling sc v0.2.3 (file:///home/tormol/p/rust/tiny-rust-executable/syscall.rs)
 
-+ rustc tinyrust.rs -O -C relocation-model=static -L syscall.rs/target/release
-++ grep '.o$'
++ rustc tinyrust.rs --crate-type lib -L syscall.rs/target/release -C relocation-model=static -O -C opt-level=z
 ++ ar t libtinyrust.rlib
+++ grep '.o$'
 + OBJECT=tinyrust.tinyrust.3a1fbbbh-cgu.0.rcgu.o
 + ar x libtinyrust.rlib tinyrust.tinyrust.3a1fbbbh-cgu.0.rcgu.o
 + objdump -dr tinyrust.tinyrust.3a1fbbbh-cgu.0.rcgu.o
@@ -27,23 +28,25 @@ tinyrust.tinyrust.3a1fbbbh-cgu.0.rcgu.o:     file format elf64-x86-64
 Disassembly of section .text.main:
 
 0000000000000000 <main>:
-   0:	bf 01 00 00 00       	mov    $0x1,%edi
-   5:	be 08 00 40 00       	mov    $0x400008,%esi
-   a:	ba 07 00 00 00       	mov    $0x7,%edx
-   f:	b8 01 00 00 00       	mov    $0x1,%eax
-  14:	0f 05                	syscall 
-  16:	31 ff                	xor    %edi,%edi
-  18:	b8 3c 00 00 00       	mov    $0x3c,%eax
-  1d:	0f 05                	syscall 
-  1f:	0f 0b                	ud2    
+   0:	6a 07                	pushq  $0x7
+   2:	5a                   	pop    %rdx
+   3:	6a 01                	pushq  $0x1
+   5:	58                   	pop    %rax
+   6:	be 08 00 40 00       	mov    $0x400008,%esi
+   b:	48 89 c7             	mov    %rax,%rdi
+   e:	0f 05                	syscall 
+  10:	6a 3c                	pushq  $0x3c
+  12:	58                   	pop    %rax
+  13:	31 ff                	xor    %edi,%edi
+  15:	0f 05                	syscall 
+  17:	0f 0b                	ud2    
 + echo
 
-
 + ld --gc-sections -e main -T script.ld -o payload tinyrust.tinyrust.3a1fbbbh-cgu.0.rcgu.o
 + objcopy -j combined -O binary payload payload.bin
 ++ nm --format=posix payload
-++ awk '{print $3}'
 ++ grep '^main '
+++ awk '{print $3}'
 + ENTRY=0000000000400070
 + nasm -f bin -o tinyrust -D entry=0x0000000000400070 elf.s
 + chmod +x tinyrust
@@ -53,14 +56,13 @@ Disassembly of section .text.main:
 00000020  38 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00  |8...............|
 00000030  00 00 00 00 38 00 38 00  01 00 00 00 07 00 00 00  |....8.8.........|
 00000040  00 00 00 00 00 00 00 00  00 00 40 00 00 00 00 00  |..........@.....|
-00000050  00 00 40 00 00 00 00 00  91 00 00 00 00 00 00 00  |..@.............|
-00000060  91 00 00 00 00 00 00 00  00 10 00 00 00 00 00 00  |................|
-00000070  bf 01 00 00 00 be 08 00  40 00 ba 07 00 00 00 b8  |........@.......|
-00000080  01 00 00 00 0f 05 31 ff  b8 3c 00 00 00 0f 05 0f  |......1..<......|
-00000090  0b                                                |.|
-00000091
+00000050  00 00 40 00 00 00 00 00  89 00 00 00 00 00 00 00  |..@.............|
+00000060  89 00 00 00 00 00 00 00  00 10 00 00 00 00 00 00  |................|
+00000070  6a 07 5a 6a 01 58 be 08  00 40 00 48 89 c7 0f 05  |j.Zj.X...@.H....|
+00000080  6a 3c 58 31 ff 0f 05 0f  0b                       |j<X1.....|
+00000089
 + wc -c tinyrust
-145 tinyrust
+137 tinyrust
 
 $ ./tinyrust
 Hello!

build.sh

@@ -20,8 +20,10 @@ fi
 set -x
 
 rustc tinyrust.rs --crate-type lib \
-    -O -C relocation-model=static \
-    -L syscall.rs/target/release
+    -L syscall.rs/target/release \
+    -C relocation-model=static \
+    -O \
+    -C opt-level=z
 
 # tinyrust.tinyrust.3a1fbbbh-cgu.0.rcgu.o
 OBJECT=$(ar t libtinyrust.rlib | grep '.o$')