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Auto merge of #121421 - saethlin:smarter-mono, r=oli-obk

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Avoid lowering code under dead SwitchInt targets

The objective of this PR is to detect and eliminate code which is guarded by an `if false`, even if that `false` is a constant which is not known until monomorphization, or is `intrinsics::debug_assertions()`.

The effect of this is that we generate no LLVM IR the standard library's unsafe preconditions, when they are compiled in a build where they should be immediately optimized out. This mono-time optimization ensures that builds which disable debug assertions do not grow a linkage requirement against `core`, which compiler-builtins currently needs: https://github.com/rust-lang/rust/issues/121552

This revives the codegen side of https://github.com/rust-lang/rust/pull/91222 as planned in https://github.com/rust-lang/rust/issues/120848.
This commit is contained in:
bors 2024-03-13 05:07:26 +00:00
commit 5a6c1aa2bc
6 changed files with 214 additions and 4 deletions
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10
compiler/rustc_codegen_ssa/src/mir/block.rs
View file

@ -1237,6 +1237,16 @@ impl<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>> FunctionCx<'a, 'tcx, Bx> {
}
}
pub fn codegen_block_as_unreachable(&mut self, bb: mir::BasicBlock) {
let llbb = match self.try_llbb(bb) {
Some(llbb) => llbb,
None => return,
};
let bx = &mut Bx::build(self.cx, llbb);
debug!("codegen_block_as_unreachable({:?})", bb);
bx.unreachable();
}
fn codegen_terminator(
&mut self,
bx: &mut Bx,

11
compiler/rustc_codegen_ssa/src/mir/mod.rs
View file

@ -256,13 +256,22 @@ pub fn codegen_mir<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>>(
// Apply debuginfo to the newly allocated locals.
fx.debug_introduce_locals(&mut start_bx);
let reachable_blocks = mir.reachable_blocks_in_mono(cx.tcx(), instance);
// The builders will be created separately for each basic block at `codegen_block`.
// So drop the builder of `start_llbb` to avoid having two at the same time.
drop(start_bx);
// Codegen the body of each block using reverse postorder
for (bb, _) in traversal::reverse_postorder(mir) {
fx.codegen_block(bb);
if reachable_blocks.contains(bb) {
fx.codegen_block(bb);
} else {
// This may have references to things we didn't monomorphize, so we
// don't actually codegen the body. We still create the block so
// terminators in other blocks can reference it without worry.
fx.codegen_block_as_unreachable(bb);
}
}
}