Auto merge of #133899 - scottmcm:strip-mir-debuginfo, r=oli-obk

We don't need `NonNull::as_ptr` debuginfo

In order to stop pessimizing the use of local variables in core, skip debug info for MIR temporaries in tiny (single-BB) functions.

For functions as simple as this -- `Pin::new`, etc -- nobody every actually wants debuginfo for them in the first place.  They're more like intrinsics than real functions, and stepping over them is good.

compiler/rustc_mir_transform/src/lib.rs

@@ -189,6 +189,7 @@ declare_passes! {
     mod simplify_comparison_integral : SimplifyComparisonIntegral;
     mod single_use_consts : SingleUseConsts;
     mod sroa : ScalarReplacementOfAggregates;
+    mod strip_debuginfo : StripDebugInfo;
     mod unreachable_enum_branching : UnreachableEnumBranching;
     mod unreachable_prop : UnreachablePropagation;
     mod validate : Validator;
@@ -699,6 +700,8 @@ fn run_optimization_passes<'tcx>(tcx: TyCtxt<'tcx>, body: &mut Body<'tcx>) {
             &o1(simplify_branches::SimplifyConstCondition::Final),
             &o1(remove_noop_landing_pads::RemoveNoopLandingPads),
             &o1(simplify::SimplifyCfg::Final),
+            // After the last SimplifyCfg, because this wants one-block functions.
+            &strip_debuginfo::StripDebugInfo,
             &copy_prop::CopyProp,
             &dead_store_elimination::DeadStoreElimination::Final,
             &nrvo::RenameReturnPlace,

compiler/rustc_mir_transform/src/strip_debuginfo.rs

@@ -0,0 +1,34 @@
+use rustc_middle::mir::*;
+use rustc_middle::ty::TyCtxt;
+use rustc_session::config::MirStripDebugInfo;
+
+/// Conditionally remove some of the VarDebugInfo in MIR.
+///
+/// In particular, stripping non-parameter debug info for tiny, primitive-like
+/// methods in core saves work later, and nobody ever wanted to use it anyway.
+pub(super) struct StripDebugInfo;
+
+impl<'tcx> crate::MirPass<'tcx> for StripDebugInfo {
+    fn is_enabled(&self, sess: &rustc_session::Session) -> bool {
+        sess.opts.unstable_opts.mir_strip_debuginfo != MirStripDebugInfo::None
+    }
+
+    fn run_pass(&self, tcx: TyCtxt<'tcx>, body: &mut Body<'tcx>) {
+        match tcx.sess.opts.unstable_opts.mir_strip_debuginfo {
+            MirStripDebugInfo::None => return,
+            MirStripDebugInfo::AllLocals => {}
+            MirStripDebugInfo::LocalsInTinyFunctions
+                if let TerminatorKind::Return { .. } =
+                    body.basic_blocks[START_BLOCK].terminator().kind => {}
+            MirStripDebugInfo::LocalsInTinyFunctions => return,
+        }
+
+        body.var_debug_info.retain(|vdi| {
+            matches!(
+                vdi.value,
+                VarDebugInfoContents::Place(place)
+                    if place.local.as_usize() <= body.arg_count && place.local != RETURN_PLACE,
+            )
+        });
+    }
+}

compiler/rustc_session/src/config.rs

@@ -472,6 +472,13 @@ impl ToString for DebugInfoCompression {
     }
 }
 
+#[derive(Clone, Copy, Debug, PartialEq, Hash)]
+pub enum MirStripDebugInfo {
+    None,
+    LocalsInTinyFunctions,
+    AllLocals,
+}
+
 /// Split debug-information is enabled by `-C split-debuginfo`, this enum is only used if split
 /// debug-information is enabled (in either `Packed` or `Unpacked` modes), and the platform
 /// uses DWARF for debug-information.
@@ -2900,10 +2907,10 @@ pub(crate) mod dep_tracking {
         BranchProtection, CFGuard, CFProtection, CollapseMacroDebuginfo, CoverageOptions,
         CrateType, DebugInfo, DebugInfoCompression, ErrorOutputType, FmtDebug, FunctionReturn,
         InliningThreshold, InstrumentCoverage, InstrumentXRay, LinkerPluginLto, LocationDetail,
-        LtoCli, NextSolverConfig, OomStrategy, OptLevel, OutFileName, OutputType, OutputTypes,
-        PatchableFunctionEntry, Polonius, RemapPathScopeComponents, ResolveDocLinks,
-        SourceFileHashAlgorithm, SplitDwarfKind, SwitchWithOptPath, SymbolManglingVersion,
-        WasiExecModel,
+        LtoCli, MirStripDebugInfo, NextSolverConfig, OomStrategy, OptLevel, OutFileName,
+        OutputType, OutputTypes, PatchableFunctionEntry, Polonius, RemapPathScopeComponents,
+        ResolveDocLinks, SourceFileHashAlgorithm, SplitDwarfKind, SwitchWithOptPath,
+        SymbolManglingVersion, WasiExecModel,
     };
     use crate::lint;
     use crate::utils::NativeLib;
@@ -2971,6 +2978,7 @@ pub(crate) mod dep_tracking {
         LtoCli,
         DebugInfo,
         DebugInfoCompression,
+        MirStripDebugInfo,
         CollapseMacroDebuginfo,
         UnstableFeatures,
         NativeLib,

compiler/rustc_session/src/options.rs

@@ -391,6 +391,8 @@ mod desc {
     pub(crate) const parse_cfprotection: &str = "`none`|`no`|`n` (default), `branch`, `return`, or `full`|`yes`|`y` (equivalent to `branch` and `return`)";
     pub(crate) const parse_debuginfo: &str = "either an integer (0, 1, 2), `none`, `line-directives-only`, `line-tables-only`, `limited`, or `full`";
     pub(crate) const parse_debuginfo_compression: &str = "one of `none`, `zlib`, or `zstd`";
+    pub(crate) const parse_mir_strip_debuginfo: &str =
+        "one of `none`, `locals-in-tiny-functions`, or `all-locals`";
     pub(crate) const parse_collapse_macro_debuginfo: &str = "one of `no`, `external`, or `yes`";
     pub(crate) const parse_strip: &str = "either `none`, `debuginfo`, or `symbols`";
     pub(crate) const parse_linker_flavor: &str = ::rustc_target::spec::LinkerFlavorCli::one_of();
@@ -925,6 +927,16 @@ pub mod parse {
         true
     }
 
+    pub(crate) fn parse_mir_strip_debuginfo(slot: &mut MirStripDebugInfo, v: Option<&str>) -> bool {
+        match v {
+            Some("none") => *slot = MirStripDebugInfo::None,
+            Some("locals-in-tiny-functions") => *slot = MirStripDebugInfo::LocalsInTinyFunctions,
+            Some("all-locals") => *slot = MirStripDebugInfo::AllLocals,
+            _ => return false,
+        };
+        true
+    }
+
     pub(crate) fn parse_linker_flavor(slot: &mut Option<LinkerFlavorCli>, v: Option<&str>) -> bool {
         match v.and_then(LinkerFlavorCli::from_str) {
             Some(lf) => *slot = Some(lf),
@@ -1893,6 +1905,8 @@ options! {
     #[rustc_lint_opt_deny_field_access("use `Session::mir_opt_level` instead of this field")]
     mir_opt_level: Option<usize> = (None, parse_opt_number, [TRACKED],
         "MIR optimization level (0-4; default: 1 in non optimized builds and 2 in optimized builds)"),
+    mir_strip_debuginfo: MirStripDebugInfo = (MirStripDebugInfo::None, parse_mir_strip_debuginfo, [TRACKED],
+        "Whether to remove some of the MIR debug info from methods.  Default: None"),
     move_size_limit: Option<usize> = (None, parse_opt_number, [TRACKED],
         "the size at which the `large_assignments` lint starts to be emitted"),
     mutable_noalias: bool = (true, parse_bool, [TRACKED],