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Auto merge of rust-lang#88832 - pcwalton:debug-unit-variant-fast-path…
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…, r=oli-obk

Introduce a fast path that avoids the `debug_tuple` abstraction when deriving Debug for unit-like enum variants.

The intent here is to allow LLVM to remove the switch entirely in favor of an
indexed load from a table of constant strings, which is likely what the
programmer would write in C. Unfortunately, LLVM currently doesn't perform this
optimization due to a bug, but there is [a
patch](https://reviews.llvm.org/D109565) that fixes this issue. I've verified
that, with that patch applied on top of this commit, Debug for unit-like tuple
variants becomes a load, reducing the O(n) code bloat to O(1).

Note that inlining `DebugTuple::finish()` wasn't enough to allow LLVM to
optimize the code properly; I had to avoid the abstraction entirely. Not using
the abstraction is likely better for compile time anyway.

Part of rust-lang#88793.

r? `@oli-obk`
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bors committed Sep 17, 2021
2 parents e366210 + 79bc538 commit 78a46ef
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Showing 2 changed files with 18 additions and 3 deletions.
20 changes: 17 additions & 3 deletions compiler/rustc_builtin_macros/src/deriving/debug.rs
Original file line number Diff line number Diff line change
Expand Up @@ -65,15 +65,29 @@ fn show_substructure(cx: &mut ExtCtxt<'_>, span: Span, substr: &Substructure<'_>
// We want to make sure we have the ctxt set so that we can use unstable methods
let span = cx.with_def_site_ctxt(span);
let name = cx.expr_lit(span, ast::LitKind::Str(ident.name, ast::StrStyle::Cooked));
let fmt = substr.nonself_args[0].clone();

// Special fast path for unit variants. In the common case of an enum that is entirely unit
// variants (i.e. a C-like enum), this fast path allows LLVM to eliminate the entire switch in
// favor of a lookup table.
if let ast::VariantData::Unit(..) = vdata {
let fn_path_write_str = cx.std_path(&[sym::fmt, sym::Formatter, sym::write_str]);
let expr = cx.expr_call_global(span, fn_path_write_str, vec![fmt, name]);
let stmts = vec![cx.stmt_expr(expr)];
let block = cx.block(span, stmts);
return cx.expr_block(block);
}

let builder = Ident::new(sym::debug_trait_builder, span);
let builder_expr = cx.expr_ident(span, builder);

let fmt = substr.nonself_args[0].clone();

let mut stmts = Vec::with_capacity(fields.len() + 2);
let fn_path_finish;
match vdata {
ast::VariantData::Tuple(..) | ast::VariantData::Unit(..) => {
ast::VariantData::Unit(..) => {
cx.span_bug(span, "unit variants should have been handled above");
}
ast::VariantData::Tuple(..) => {
// tuple struct/"normal" variant
let fn_path_debug_tuple = cx.std_path(&[sym::fmt, sym::Formatter, sym::debug_tuple]);
let expr = cx.expr_call_global(span, fn_path_debug_tuple, vec![fmt, name]);
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1 change: 1 addition & 0 deletions compiler/rustc_span/src/symbol.rs
Original file line number Diff line number Diff line change
Expand Up @@ -1417,6 +1417,7 @@ symbols! {
wrapping_sub,
wreg,
write_bytes,
write_str,
x87_reg,
xer,
xmm_reg,
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