document this feature in the unstable book

Author: japaric

src/doc/unstable-book/src/compiler-flags/emit-stack-sizes.md

@@ -0,0 +1,153 @@
+# `emit-stack-sizes`
+
+The tracking issue for this feature is: [#54192]
+
+[#54192]: https://github.com/rust-lang/rust/issues/54192
+
+------------------------
+
+The rustc flag `-Z emit-stack-sizes` makes LLVM emit stack size metadata.
+
+Consider this crate:
+
+```
+#![crate_type = "lib"]
+
+use std::ptr;
+
+pub fn foo() {
+    // this function doesn't use the stack
+}
+
+pub fn bar() {
+    let xs = [0u32; 2];
+
+    // force LLVM to allocate `xs` on the stack
+    unsafe { ptr::read_volatile(&xs.as_ptr()); }
+}
+```
+
+Using the `-Z emit-stack-sizes` flag produces extra linker sections in the
+output *object file*.
+
+``` console
+$ rustc -C opt-level=3 --emit=obj foo.rs
+
+$ size -A foo.o
+foo.o  :
+section                                 size   addr
+.text                                      0      0
+.text._ZN3foo3foo17he211d7b4a3a0c16eE      1      0
+.text._ZN3foo3bar17h1acb594305f70c2eE     22      0
+.note.GNU-stack                            0      0
+.eh_frame                                 72      0
+Total                                     95
+
+$ rustc -C opt-level=3 --emit=obj -Z emit-stack-sizes foo.rs
+
+$ size -A foo.o
+foo.o  :
+section                                 size   addr
+.text                                      0      0
+.text._ZN3foo3foo17he211d7b4a3a0c16eE      1      0
+.stack_sizes                               9      0
+.text._ZN3foo3bar17h1acb594305f70c2eE     22      0
+.stack_sizes                               9      0
+.note.GNU-stack                            0      0
+.eh_frame                                 72      0
+Total                                    113
+```
+
+As of LLVM 7.0 the data will be written into a section named `.stack_sizes` and
+the format is "an array of pairs of function symbol values (pointer size) and
+stack sizes (unsigned LEB128)".
+
+``` console
+$ objdump -d foo.o
+
+foo.o:     file format elf64-x86-64
+
+Disassembly of section .text._ZN3foo3foo17he211d7b4a3a0c16eE:
+
+0000000000000000 <_ZN3foo3foo17he211d7b4a3a0c16eE>:
+   0:   c3                      retq
+
+Disassembly of section .text._ZN3foo3bar17h1acb594305f70c2eE:
+
+0000000000000000 <_ZN3foo3bar17h1acb594305f70c2eE>:
+   0:   48 83 ec 10             sub    $0x10,%rsp
+   4:   48 8d 44 24 08          lea    0x8(%rsp),%rax
+   9:   48 89 04 24             mov    %rax,(%rsp)
+   d:   48 8b 04 24             mov    (%rsp),%rax
+  11:   48 83 c4 10             add    $0x10,%rsp
+  15:   c3                      retq
+
+$ objdump -s -j .stack_sizes foo.o
+
+foo.o:     file format elf64-x86-64
+
+Contents of section .stack_sizes:
+ 0000 00000000 00000000 00                 .........
+Contents of section .stack_sizes:
+ 0000 00000000 00000000 10                 .........
+```
+
+It's important to note that linkers will discard this linker section by default.
+To preserve the section you can use a linker script like the one shown below.
+
+``` text
+/* file: keep-stack-sizes.x */
+SECTIONS
+{
+  .stack_sizes :
+  {
+    KEEP(*(.stack_sizes));
+  }
+}
+```
+
+The linker script must be passed to the linker using a rustc flag like `-C
+link-arg`.
+
+```
+// file: src/main.rs
+use std::ptr;
+
+#[inline(never)]
+fn main() {
+    let xs = [0u32; 2];
+
+    // force LLVM to allocate `xs` on the stack
+    unsafe { ptr::read_volatile(&xs.as_ptr()); }
+}
+```
+
+``` console
+$ RUSTFLAGS="-Z emit-stack-sizes" cargo build --release
+
+$ size -A target/release/hello | grep stack_sizes || echo section was not found
+section was not found
+
+$ RUSTFLAGS="-Z emit-stack-sizes" cargo rustc --release -- \
+    -C link-arg=-Wl,-Tkeep-stack-sizes.x \
+    -C link-arg=-N
+
+$ size -A target/release/hello | grep stack_sizes
+.stack_sizes                               90   205368
+
+$ objdump -s -j .stack_sizes target/release/hello
+
+target/release/hello:     file format elf64-x86-64
+
+Contents of section .stack_sizes:
+ 32238 c0040000 00000000 08f00400 00000000  ................
+ 32248 00080005 00000000 00000810 05000000  ................
+ 32258 00000000 20050000 00000000 10400500  .... ........@..
+ 32268 00000000 00087005 00000000 00000080  ......p.........
+ 32278 05000000 00000000 90050000 00000000  ................
+ 32288 00a00500 00000000 0000               ..........
+```
+
+> Author note: I'm not entirely sure why, in *this* case, `-N` is required in
+> addition to `-Tkeep-stack-sizes.x`. For example, it's not required when
+> producing statically linked files for the ARM Cortex-M architecture.