rust/tests/debuginfo/limited-debuginfo.rs

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// ignore-lldb
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// ignore-gdb // Test temporarily ignored due to debuginfo tests being disabled, see PR 47155
// compile-flags:-C debuginfo=1
// Make sure functions have proper names
// gdb-command:info functions
// gdbg-check:[...]void[...]main([...]);
// gdbr-check:fn limited_debuginfo::main();
// gdbg-check:[...]void[...]some_function([...]);
// gdbr-check:fn limited_debuginfo::some_function();
// gdbg-check:[...]void[...]some_other_function([...]);
// gdbr-check:fn limited_debuginfo::some_other_function();
// gdbg-check:[...]void[...]zzz([...]);
// gdbr-check:fn limited_debuginfo::zzz();
// gdb-command:run
// Make sure there is no information about locals
// gdb-command:info locals
// gdb-check:No locals.
// gdb-command:continue
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#![allow(unused_variables)]
#![feature(omit_gdb_pretty_printer_section)]
#![omit_gdb_pretty_printer_section]
struct Struct {
a: i64,
b: i32
}
fn main() {
some_function(101, 202);
rustc: Enable -f{function,data}-sections The compiler has previously been producing binaries on the order of 1.8MB for hello world programs "fn main() {}". This is largely a result of the compilation model used by compiling entire libraries into a single object file and because static linking is favored by default. When linking, linkers will pull in the entire contents of an object file if any symbol from the object file is used. This means that if any symbol from a rust library is used, the entire library is pulled in unconditionally, regardless of whether the library is used or not. Traditional C/C++ projects do not normally encounter these large executable problems because their archives (rust's rlibs) are composed of many objects. Because of this, linkers can eliminate entire objects from being in the final executable. With rustc, however, the linker does not have the opportunity to leave out entire object files. In order to get similar benefits from dead code stripping at link time, this commit enables the -ffunction-sections and -fdata-sections flags in LLVM, as well as passing --gc-sections to the linker *by default*. This means that each function and each global will be placed into its own section, allowing the linker to GC all unused functions and data symbols. By enabling these flags, rust is able to generate much smaller binaries default. On linux, a hello world binary went from 1.8MB to 597K (a 67% reduction in size). The output size of dynamic libraries remained constant, but the output size of rlibs increased, as seen below: libarena - 2.27% bigger ( 292872 => 299508) libcollections - 0.64% bigger ( 6765884 => 6809076) libflate - 0.83% bigger ( 186516 => 188060) libfourcc - 14.71% bigger ( 307290 => 352498) libgetopts - 4.42% bigger ( 761468 => 795102) libglob - 2.73% bigger ( 899932 => 924542) libgreen - 9.63% bigger ( 1281718 => 1405124) libhexfloat - 13.88% bigger ( 333738 => 380060) liblibc - 10.79% bigger ( 551280 => 610736) liblog - 10.93% bigger ( 218208 => 242060) libnative - 8.26% bigger ( 1362096 => 1474658) libnum - 2.34% bigger ( 2583400 => 2643916) librand - 1.72% bigger ( 1608684 => 1636394) libregex - 6.50% bigger ( 1747768 => 1861398) librustc - 4.21% bigger (151820192 => 158218924) librustdoc - 8.96% bigger ( 13142604 => 14320544) librustuv - 4.13% bigger ( 4366896 => 4547304) libsemver - 2.66% bigger ( 396166 => 406686) libserialize - 1.91% bigger ( 6878396 => 7009822) libstd - 3.59% bigger ( 39485286 => 40902218) libsync - 3.95% bigger ( 1386390 => 1441204) libsyntax - 4.96% bigger ( 35757202 => 37530798) libterm - 13.99% bigger ( 924580 => 1053902) libtest - 6.04% bigger ( 2455720 => 2604092) libtime - 2.84% bigger ( 1075708 => 1106242) liburl - 6.53% bigger ( 590458 => 629004) libuuid - 4.63% bigger ( 326350 => 341466) libworkcache - 8.45% bigger ( 1230702 => 1334750) This increase in size is a result of encoding many more section names into each object file (rlib). These increases are moderate enough that this change seems worthwhile to me, due to the drastic improvements seen in the final artifacts. The overall increase of the stage2 target folder (not the size of an install) went from 337MB to 348MB (3% increase). Additionally, linking is generally slower when executed with all these new sections plus the --gc-sections flag. The stage0 compiler takes 1.4s to link the `rustc` binary, where the stage1 compiler takes 1.9s to link the binary. Three megabytes are shaved off the binary. I found this increase in link time to be acceptable relative to the benefits of code size gained. This commit only enables --gc-sections for *executables*, not dynamic libraries. LLVM does all the heavy lifting when producing an object file for a dynamic library, so there is little else for the linker to do (remember that we only have one object file). I conducted similar experiments by putting a *module's* functions and data symbols into its own section (granularity moved to a module level instead of a function/static level). The size benefits of a hello world were seen to be on the order of 400K rather than 1.2MB. It seemed that enough benefit was gained using ffunction-sections that this route was less desirable, despite the lesser increases in binary rlib size.
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some_other_function(1, 2);
}
fn zzz() {()}
fn some_function(a: isize, b: isize) {
let some_variable = Struct { a: 11, b: 22 };
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let some_other_variable = 23;
for x in 0..1 {
zzz(); // #break
}
}
fn some_other_function(a: isize, b: isize) -> bool { true }