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Add Option to Force Unwind Tables #69984

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12 changes: 12 additions & 0 deletions src/doc/rustc/src/codegen-options/index.md
Original file line number Diff line number Diff line change
@@ -98,6 +98,18 @@ values:
The default behaviour, if frame pointers are not force-enabled, depends on the
target.

## force-unwind-tables

This flag forces the generation of unwind tables. It takes one of the following
values:

* `y`, `yes`, `on`, or no value: Unwind tables are forced to be generated.
* `n`, `no`, or `off`: Unwind tables are not forced to be generated. If unwind
tables are required by the target or `-C panic=unwind`, an error will be
emitted.

The default if not specified depends on the target.

## incremental

This flag allows you to enable incremental compilation, which allows `rustc`
2 changes: 1 addition & 1 deletion src/librustc_codegen_llvm/allocator.rs
Original file line number Diff line number Diff line change
@@ -54,7 +54,7 @@ pub(crate) unsafe fn codegen(tcx: TyCtxt<'_>, mods: &mut ModuleLlvm, kind: Alloc
if tcx.sess.target.target.options.default_hidden_visibility {
llvm::LLVMRustSetVisibility(llfn, llvm::Visibility::Hidden);
}
if tcx.sess.target.target.options.requires_uwtable {
if tcx.sess.must_emit_unwind_tables() {
attributes::emit_uwtable(llfn, true);
}

5 changes: 1 addition & 4 deletions src/librustc_codegen_llvm/attributes.rs
Original file line number Diff line number Diff line change
@@ -13,7 +13,6 @@ use rustc_middle::ty::query::Providers;
use rustc_middle::ty::{self, TyCtxt};
use rustc_session::config::{OptLevel, Sanitizer};
use rustc_session::Session;
use rustc_target::spec::PanicStrategy;

use crate::attributes;
use crate::llvm::AttributePlace::Function;
@@ -271,9 +270,7 @@ pub fn from_fn_attrs(cx: &CodegenCx<'ll, 'tcx>, llfn: &'ll Value, instance: ty::
//
// You can also find more info on why Windows is whitelisted here in:
// https://bugzilla.mozilla.org/show_bug.cgi?id=1302078
if cx.sess().panic_strategy() == PanicStrategy::Unwind
|| cx.sess().target.target.options.requires_uwtable
{
if cx.sess().must_emit_unwind_tables() {
attributes::emit_uwtable(llfn, true);
}

1 change: 1 addition & 0 deletions src/librustc_interface/tests.rs
Original file line number Diff line number Diff line change
@@ -415,6 +415,7 @@ fn test_codegen_options_tracking_hash() {
tracked!(debuginfo, 0xdeadbeef);
tracked!(embed_bitcode, false);
tracked!(force_frame_pointers, Some(false));
tracked!(force_unwind_tables, Some(true));
tracked!(inline_threshold, Some(0xf007ba11));
tracked!(linker_plugin_lto, LinkerPluginLto::LinkerPluginAuto);
tracked!(llvm_args, vec![String::from("1"), String::from("2")]);
2 changes: 2 additions & 0 deletions src/librustc_session/options.rs
Original file line number Diff line number Diff line change
@@ -668,6 +668,8 @@ options! {CodegenOptions, CodegenSetter, basic_codegen_options,
"extra data to put in each output filename"),
force_frame_pointers: Option<bool> = (None, parse_opt_bool, [TRACKED],
"force use of the frame pointers"),
force_unwind_tables: Option<bool> = (None, parse_opt_bool, [TRACKED],
"force use of unwind tables"),
incremental: Option<String> = (None, parse_opt_string, [UNTRACKED],
"enable incremental compilation"),
inline_threshold: Option<usize> = (None, parse_opt_uint, [TRACKED],
44 changes: 44 additions & 0 deletions src/librustc_session/session.rs
Original file line number Diff line number Diff line change
@@ -601,6 +601,33 @@ impl Session {
}
}

pub fn must_emit_unwind_tables(&self) -> bool {
// This is used to control the emission of the `uwtable` attribute on
// LLVM functions.
//
// At the very least, unwind tables are needed when compiling with
// `-C panic=unwind`.
//
// On some targets (including windows), however, exceptions include
// other events such as illegal instructions, segfaults, etc. This means
// that on Windows we end up still needing unwind tables even if the `-C
// panic=abort` flag is passed.
//
// You can also find more info on why Windows needs unwind tables in:
// https://bugzilla.mozilla.org/show_bug.cgi?id=1302078
//
// If a target requires unwind tables, then they must be emitted.
// Otherwise, we can defer to the `-C force-unwind-tables=<yes/no>`
// value, if it is provided, or disable them, if not.
if self.panic_strategy() == PanicStrategy::Unwind {
true
} else if self.target.target.options.requires_uwtable {
true
} else {
self.opts.cg.force_unwind_tables.unwrap_or(false)
}
}

/// Returns the symbol name for the registrar function,
/// given the crate `Svh` and the function `DefIndex`.
pub fn generate_plugin_registrar_symbol(&self, disambiguator: CrateDisambiguator) -> String {
@@ -1178,6 +1205,23 @@ fn validate_commandline_args_with_session_available(sess: &Session) {
}
}

// Unwind tables cannot be disabled if the target requires them.
if let Some(include_uwtables) = sess.opts.cg.force_unwind_tables {
if sess.panic_strategy() == PanicStrategy::Unwind && !include_uwtables {
sess.err(
"panic=unwind requires unwind tables, they cannot be disabled \
with `-C force-unwind-tables=no`.",
);
}

if sess.target.target.options.requires_uwtable && !include_uwtables {
sess.err(
"target requires unwind tables, they cannot be disabled with \
`-C force-unwind-tables=no`.",
);
}
}

// PGO does not work reliably with panic=unwind on Windows. Let's make it
// an error to combine the two for now. It always runs into an assertions
// if LLVM is built with assertions, but without assertions it sometimes
7 changes: 7 additions & 0 deletions src/test/codegen/force-unwind-tables.rs
Original file line number Diff line number Diff line change
@@ -0,0 +1,7 @@
// min-llvm-version 8.0
// compile-flags: -C no-prepopulate-passes -C force-unwind-tables=y

#![crate_type="lib"]

// CHECK: attributes #{{.*}} uwtable
pub fn foo() {}
10 changes: 10 additions & 0 deletions src/test/compile-fail/unwind-tables-panic-required.rs
Original file line number Diff line number Diff line change
@@ -0,0 +1,10 @@
// Tests that the compiler errors if the user tries to turn off unwind tables
// when they are required.
//
// compile-flags: -C panic=unwind -C force-unwind-tables=no
// ignore-tidy-linelength
//
// error-pattern: panic=unwind requires unwind tables, they cannot be disabled with `-C force-unwind-tables=no`.

pub fn main() {
}
11 changes: 11 additions & 0 deletions src/test/compile-fail/unwind-tables-target-required.rs
Original file line number Diff line number Diff line change
@@ -0,0 +1,11 @@
// Tests that the compiler errors if the user tries to turn off unwind tables
// when they are required.
//
// only-x86_64-windows-msvc
// compile-flags: -C force-unwind-tables=no
// ignore-tidy-linelength
//
// error-pattern: target requires unwind tables, they cannot be disabled with `-C force-unwind-tables=no`.

pub fn main() {
}