From 2f7b4d91e21863cd7e9ffc24d18299db432639c5 Mon Sep 17 00:00:00 2001
From: Alex Brachet <abrachet@google.com>
Date: Mon, 10 Oct 2022 20:13:10 +0000
Subject: [PATCH 1/2] Don't internalize __llvm_profile_counter_bias
Currently, LLVM profiling runtime counter relocation cannot be
used by rust during LTO because symbols are being internalized
before all symbol information is known.
This mode makes LLVM emit a __llvm_profile_counter_bias symbol
which is referenced by the profiling initialization, which itself
is pulled in by the rust driver here [1].
It is enabled with -Cllvm-args=-runtime-counter-relocation for
platforms which are opt-in to this mode like Linux. On these
platforms there will be no link error, rather just surprising
behavior for a user which request runtime counter relocation.
The profiling runtime will not see that symbol go on as if it
were never there. On Fuchsia, the profiling runtime must have
this symbol which will cause a hard link error.
As an aside, I don't have enough context as to why rust's LTO
model is how it is. AFAICT, the internalize pass is only safe
to run at link time when all symbol information is actually
known, this being an example as to why. I think special casing
this symbol as a known one that LLVM can emit which should not
have it's visbility de-escalated should be fine given how
seldom this pattern of defining an undefined symbol to get
initilization code pulled in is. From a quick grep,
__llvm_profile_runtime is the only symbol that rustc does this
for.
[1] https://github.com/rust-lang/rust/blob/0265a3e93bf1b89d97cae113ed214954d5c35e22/compiler/rustc_codegen_ssa/src/back/linker.rs#L598
---
.../rustc_codegen_ssa/src/back/symbol_export.rs | 15 +++++++++++----
src/test/codegen/pgo-counter-bias.rs | 9 +++++++++
2 files changed, 20 insertions(+), 4 deletions(-)
create mode 100644 src/test/codegen/pgo-counter-bias.rs
diff --git a/compiler/rustc_codegen_ssa/src/back/symbol_export.rs b/compiler/rustc_codegen_ssa/src/back/symbol_export.rs
index 8d7e2c5cf3939..341b28424e40b 100644
--- a/compiler/rustc_codegen_ssa/src/back/symbol_export.rs
+++ b/compiler/rustc_codegen_ssa/src/back/symbol_export.rs
@@ -212,10 +212,17 @@ fn exported_symbols_provider_local<'tcx>(
// These are weak symbols that point to the profile version and the
// profile name, which need to be treated as exported so LTO doesn't nix
// them.
- const PROFILER_WEAK_SYMBOLS: [&str; 2] =
- ["__llvm_profile_raw_version", "__llvm_profile_filename"];
-
- symbols.extend(PROFILER_WEAK_SYMBOLS.iter().map(|sym| {
+ // __llvm_profile_counter_bias is not weak, but is only referenced by the
+ // profiling initialization runtime, which is itself pulled in by an
+ // undefined reference to __llvm_profile_runtime. This happens at link time
+ // and thus this symbol cannot be internalized until then.
+ const PROFILER_DEFAULT_VIS_SYMBOLS: [&str; 3] = [
+ "__llvm_profile_raw_version",
+ "__llvm_profile_filename",
+ "__llvm_profile_counter_bias",
+ ];
+
+ symbols.extend(PROFILER_DEFAULT_VIS_SYMBOLS.iter().map(|sym| {
let exported_symbol = ExportedSymbol::NoDefId(SymbolName::new(tcx, sym));
(
exported_symbol,
diff --git a/src/test/codegen/pgo-counter-bias.rs b/src/test/codegen/pgo-counter-bias.rs
new file mode 100644
index 0000000000000..b9019e9de7b22
--- /dev/null
+++ b/src/test/codegen/pgo-counter-bias.rs
@@ -0,0 +1,9 @@
+// Test that __llvm_profile_counter_bias does not get internalized by lto.
+
+// compile-flags: -Cprofile-generate -Cllvm-args=-runtime-counter-relocation -Clto=fat
+// needs-profiler-support
+// no-prefer-dynamic
+
+// CHECK: @__llvm_profile_counter_bias = {{.*}}global
+
+pub fn main() {}
From f6d7f0807275f709adab2a3ead9c51d0f2291e9d Mon Sep 17 00:00:00 2001
From: StackDoubleFlow <ojaslandge@gmail.com>
Date: Mon, 7 Nov 2022 22:42:07 -0600
Subject: [PATCH 2/2] Issue error when `-C link-self-contained` option is used
on unsupported platforms
Document supported targets for `-C link-self-contained`
Move `LinkSelfContainedDefault::True` from wasm_base to wasm32_wasi
---
compiler/rustc_codegen_ssa/src/back/link.rs | 3 +++
compiler/rustc_codegen_ssa/src/errors.rs | 4 ++++
compiler/rustc_error_messages/locales/en-US/codegen_ssa.ftl | 2 ++
compiler/rustc_target/src/spec/wasm32_wasi.rs | 6 +++++-
compiler/rustc_target/src/spec/wasm_base.rs | 4 ----
src/doc/rustc/src/codegen-options/index.md | 4 ++--
6 files changed, 16 insertions(+), 7 deletions(-)
diff --git a/compiler/rustc_codegen_ssa/src/back/link.rs b/compiler/rustc_codegen_ssa/src/back/link.rs
index 5a1ad792924fc..cc9367f53265a 100644
--- a/compiler/rustc_codegen_ssa/src/back/link.rs
+++ b/compiler/rustc_codegen_ssa/src/back/link.rs
@@ -1582,6 +1582,9 @@ fn detect_self_contained_mingw(sess: &Session) -> bool {
/// We only provide such support for a very limited number of targets.
fn self_contained(sess: &Session, crate_type: CrateType) -> bool {
if let Some(self_contained) = sess.opts.cg.link_self_contained {
+ if sess.target.link_self_contained == LinkSelfContainedDefault::False {
+ sess.emit_err(errors::UnsupportedLinkSelfContained);
+ }
return self_contained;
}
diff --git a/compiler/rustc_codegen_ssa/src/errors.rs b/compiler/rustc_codegen_ssa/src/errors.rs
index 36c94462b0b3e..d24579364c476 100644
--- a/compiler/rustc_codegen_ssa/src/errors.rs
+++ b/compiler/rustc_codegen_ssa/src/errors.rs
@@ -521,3 +521,7 @@ pub enum AppleSdkRootError<'a> {
#[diag(codegen_ssa_apple_sdk_error_sdk_path)]
SdkPath { sdk_name: &'a str, error: Error },
}
+
+#[derive(Diagnostic)]
+#[diag(codegen_ssa_unsupported_link_self_contained)]
+pub struct UnsupportedLinkSelfContained;
diff --git a/compiler/rustc_error_messages/locales/en-US/codegen_ssa.ftl b/compiler/rustc_error_messages/locales/en-US/codegen_ssa.ftl
index ad0d758210175..717c7816fabc2 100644
--- a/compiler/rustc_error_messages/locales/en-US/codegen_ssa.ftl
+++ b/compiler/rustc_error_messages/locales/en-US/codegen_ssa.ftl
@@ -182,3 +182,5 @@ codegen_ssa_extract_bundled_libs_write_file = failed to write file '{$rlib}': {$
codegen_ssa_unsupported_arch = unsupported arch `{$arch}` for os `{$os}`
codegen_ssa_apple_sdk_error_sdk_path = failed to get {$sdk_name} SDK path: {error}
+
+codegen_ssa_unsupported_link_self_contained = option `-C link-self-contained` is not supported on this target
diff --git a/compiler/rustc_target/src/spec/wasm32_wasi.rs b/compiler/rustc_target/src/spec/wasm32_wasi.rs
index 93a956403e50f..6f0bbf0672d44 100644
--- a/compiler/rustc_target/src/spec/wasm32_wasi.rs
+++ b/compiler/rustc_target/src/spec/wasm32_wasi.rs
@@ -72,7 +72,8 @@
//! best we can with this target. Don't start relying on too much here unless
//! you know what you're getting in to!
-use super::{crt_objects, wasm_base, Cc, LinkerFlavor, Target};
+use super::crt_objects::{self, LinkSelfContainedDefault};
+use super::{wasm_base, Cc, LinkerFlavor, Target};
pub fn target() -> Target {
let mut options = wasm_base::options();
@@ -83,6 +84,9 @@ pub fn target() -> Target {
options.pre_link_objects_self_contained = crt_objects::pre_wasi_self_contained();
options.post_link_objects_self_contained = crt_objects::post_wasi_self_contained();
+ // FIXME: Figure out cases in which WASM needs to link with a native toolchain.
+ options.link_self_contained = LinkSelfContainedDefault::True;
+
// Right now this is a bit of a workaround but we're currently saying that
// the target by default has a static crt which we're taking as a signal
// for "use the bundled crt". If that's turned off then the system's crt
diff --git a/compiler/rustc_target/src/spec/wasm_base.rs b/compiler/rustc_target/src/spec/wasm_base.rs
index 528a84a8b37cb..625d3b37c4f26 100644
--- a/compiler/rustc_target/src/spec/wasm_base.rs
+++ b/compiler/rustc_target/src/spec/wasm_base.rs
@@ -1,4 +1,3 @@
-use super::crt_objects::LinkSelfContainedDefault;
use super::{cvs, Cc, LinkerFlavor, PanicStrategy, RelocModel, TargetOptions, TlsModel};
pub fn options() -> TargetOptions {
@@ -95,9 +94,6 @@ pub fn options() -> TargetOptions {
pre_link_args,
- // FIXME: Figure out cases in which WASM needs to link with a native toolchain.
- link_self_contained: LinkSelfContainedDefault::True,
-
// This has no effect in LLVM 8 or prior, but in LLVM 9 and later when
// PIC code is implemented this has quite a drastic effect if it stays
// at the default, `pic`. In an effort to keep wasm binaries as minimal
diff --git a/src/doc/rustc/src/codegen-options/index.md b/src/doc/rustc/src/codegen-options/index.md
index f5a49410ea555..7e355b7fccfc4 100644
--- a/src/doc/rustc/src/codegen-options/index.md
+++ b/src/doc/rustc/src/codegen-options/index.md
@@ -210,8 +210,8 @@ metrics.
## link-self-contained
-On targets that support it this flag controls whether the linker will use libraries and objects
-shipped with Rust instead or those in the system.
+On `windows-gnu`, `linux-musl`, and `wasi` targets, this flag controls whether the
+linker will use libraries and objects shipped with Rust instead or those in the system.
It takes one of the following values:
* no value: rustc will use heuristic to disable self-contained mode if system has necessary tools.