Rollup merge of #124741 - nebulark:patchable-function-entries-pr, r=estebank,workingjubilee

patchable-function-entry: Add unstable compiler flag and attribute

Tracking issue: #123115

Add the -Z patchable-function-entry compiler flag and the #[patchable_function_entry(prefix_nops = m, entry_nops = n)] attribute.
Rebased and adjusted the canditate implementation to match changes in the RFC.

compiler/rustc_session/src/config.rs

@@ -2965,8 +2965,9 @@ pub(crate) mod dep_tracking {
         CrateType, DebugInfo, DebugInfoCompression, ErrorOutputType, FunctionReturn,
         InliningThreshold, InstrumentCoverage, InstrumentXRay, LinkerPluginLto, LocationDetail,
         LtoCli, NextSolverConfig, OomStrategy, OptLevel, OutFileName, OutputType, OutputTypes,
-        Polonius, RemapPathScopeComponents, ResolveDocLinks, SourceFileHashAlgorithm,
-        SplitDwarfKind, SwitchWithOptPath, SymbolManglingVersion, WasiExecModel,
+        PatchableFunctionEntry, Polonius, RemapPathScopeComponents, ResolveDocLinks,
+        SourceFileHashAlgorithm, SplitDwarfKind, SwitchWithOptPath, SymbolManglingVersion,
+        WasiExecModel,
     };
     use crate::lint;
     use crate::utils::NativeLib;
@@ -3073,6 +3074,7 @@ pub(crate) mod dep_tracking {
         OomStrategy,
         LanguageIdentifier,
         NextSolverConfig,
+        PatchableFunctionEntry,
         Polonius,
         InliningThreshold,
         FunctionReturn,
@@ -3250,6 +3252,35 @@ impl DumpMonoStatsFormat {
     }
 }
 
+/// `-Z patchable-function-entry` representation - how many nops to put before and after function
+/// entry.
+#[derive(Clone, Copy, PartialEq, Hash, Debug, Default)]
+pub struct PatchableFunctionEntry {
+    /// Nops before the entry
+    prefix: u8,
+    /// Nops after the entry
+    entry: u8,
+}
+
+impl PatchableFunctionEntry {
+    pub fn from_total_and_prefix_nops(
+        total_nops: u8,
+        prefix_nops: u8,
+    ) -> Option<PatchableFunctionEntry> {
+        if total_nops < prefix_nops {
+            None
+        } else {
+            Some(Self { prefix: prefix_nops, entry: total_nops - prefix_nops })
+        }
+    }
+    pub fn prefix(&self) -> u8 {
+        self.prefix
+    }
+    pub fn entry(&self) -> u8 {
+        self.entry
+    }
+}
+
 /// `-Zpolonius` values, enabling the borrow checker polonius analysis, and which version: legacy,
 /// or future prototype.
 #[derive(Clone, Copy, PartialEq, Hash, Debug, Default)]

compiler/rustc_session/src/options.rs

@@ -379,6 +379,7 @@ mod desc {
     pub const parse_passes: &str = "a space-separated list of passes, or `all`";
     pub const parse_panic_strategy: &str = "either `unwind` or `abort`";
     pub const parse_on_broken_pipe: &str = "either `kill`, `error`, or `inherit`";
+    pub const parse_patchable_function_entry: &str = "either two comma separated integers (total_nops,prefix_nops), with prefix_nops <= total_nops, or one integer (total_nops)";
     pub const parse_opt_panic_strategy: &str = parse_panic_strategy;
     pub const parse_oom_strategy: &str = "either `panic` or `abort`";
     pub const parse_relro_level: &str = "one of: `full`, `partial`, or `off`";
@@ -734,6 +735,32 @@ mod parse {
         true
     }
 
+    pub(crate) fn parse_patchable_function_entry(
+        slot: &mut PatchableFunctionEntry,
+        v: Option<&str>,
+    ) -> bool {
+        let mut total_nops = 0;
+        let mut prefix_nops = 0;
+
+        if !parse_number(&mut total_nops, v) {
+            let parts = v.and_then(|v| v.split_once(',')).unzip();
+            if !parse_number(&mut total_nops, parts.0) {
+                return false;
+            }
+            if !parse_number(&mut prefix_nops, parts.1) {
+                return false;
+            }
+        }
+
+        if let Some(pfe) =
+            PatchableFunctionEntry::from_total_and_prefix_nops(total_nops, prefix_nops)
+        {
+            *slot = pfe;
+            return true;
+        }
+        false
+    }
+
     pub(crate) fn parse_oom_strategy(slot: &mut OomStrategy, v: Option<&str>) -> bool {
         match v {
             Some("panic") => *slot = OomStrategy::Panic,
@@ -1859,6 +1886,8 @@ options! {
         "panic strategy for panics in drops"),
     parse_only: bool = (false, parse_bool, [UNTRACKED],
         "parse only; do not compile, assemble, or link (default: no)"),
+    patchable_function_entry: PatchableFunctionEntry = (PatchableFunctionEntry::default(), parse_patchable_function_entry, [TRACKED],
+        "nop padding at function entry"),
     plt: Option<bool> = (None, parse_opt_bool, [TRACKED],
         "whether to use the PLT when calling into shared libraries;
         only has effect for PIC code on systems with ELF binaries