Add comments about the size of the tiny executable. (#44)

Author: sunfishcode

Cargo.toml

@@ -15,7 +15,7 @@ include = ["src", "Cargo.toml", "COPYRIGHT", "LICENSE*", "/*.md"]
 
 [dependencies]
 linux-raw-sys = { version = "0.4.7", default-features = false, features = ["general", "no_std", "elf"] }
-rustix = { version = "0.38.11", default-features = false }
+rustix = { version = "0.38.13", default-features = false }
 bitflags = "2.4.0"
 memoffset = { version = "0.9.0", optional = true }
 log = { version = "0.4.14", default-features = false, optional = true }

README.md

@@ -57,8 +57,9 @@ Origin can also be used on its own, in several different configurations:
 
  - The [origin-start-lto example] is like origin-start, but builds with LTO.
 
- - The [tiny example] is like origin-start, but builds with optimization flags
-   and disables features to build a very small binary.
+ - The [tiny example] is like origin-start, but builds with optimization flags,
+   disables features, and adds an objcopy trick to produce a very small
+   binary—408 bytes on x86-64!
 
 ## Fully static linking
 

example-crates/tiny/README.md

@@ -9,23 +9,27 @@ and `.comment` sections:
 objcopy -R .eh_frame -R .comment target/release/tiny even-smaller
 ```
 
-For details on the specific optimizations performed, see the options under
-`[profile.release]` and the use of `default-features = false`, in Cargo.toml,
-and the additional link flags passed in build.rs.
+On x86_64, this produces a executable with size 408 bytes.
+
+How is this achieved?
 
 ## The optimizations
 
-First, `origin` makes much of its functionality optional, so we add
-`default-features = false` to disable things like `.init_array`/`.fini_array`
-support, thread support, and other things. We only enable the features needed
-for our minimal test program:
+First, to make this example really simple, we change it from printing a message
+to just returning the number 42.
+
+Next, `origin` makes much of its functionality optional, so we add
+`default-features = false` when declaring the `origin` dependency, to disable
+things like `.init_array`/`.fini_array` support, thread support, and other
+things that our simple example doesn't need. We only enable the features
+needed for our minimal test program:
 
 ```toml
 origin = { path = "../..", default-features = false, features = ["origin-program", "origin-start"] }
 ```
 
-Then, we enable several optimizations in the `#[profile.release]` section of
-Cargo.toml:
+Then, we add a `#[profile.release]` section to our Cargo.toml, to enable
+several optimizations:
 
 ```toml
 # Give the optimizer more lattitude to optimize and delete unneeded code.
@@ -194,6 +198,8 @@ Those first 3 instructions are origin's `_start` function. The next 5
 instructions are `origin::program::entry` and everything, including the user
 `main` function and the `exit_group` syscall inlined into it.
 
+## Optimizations not done
+
 In theory this code code be made even smaller.
 
 That first `mov $rsp,%rdi` is moving the incoming stack pointer we got from the