store Symbols in a table as well, like positions

this slightly increases the amount of memory used for any given symbol, but this
increase is more than made up for if the symbol is referenced more than once in
the EvalState that holds it. on average every symbol should be referenced at
least twice (once to introduce a binding, once to use it), so we expect no
increase in memory on average.

symbol tables are limited to 2³² entries like position tables, and similar
arguments apply to why overflow is not likely: 2³² symbols would require as many
string instances (at 24 bytes each) and map entries (at 24 bytes or more each,
assuming that the map holds on average at most one item per bucket as the docs
say). a full symbol table would require at least 192GB of memory just for
symbols, which is well out of reach. (an ofborg eval of nixpks today creates
less than a million symbols!)

1 files changed, 8 insertions, 7 deletions

diff --git a/src/libexpr/eval.hh b/src/libexpr/eval.hh
index 71d6e7e7f..59c9c4873 100644
--- a/src/libexpr/eval.hh
+++ b/src/libexpr/eval.hh
@@ -53,7 +53,8 @@ void copyContext(const Value & v, PathSet & context);
 typedef std::map<Path, StorePath> SrcToStore;
 
 
-std::ostream & operator << (std::ostream & str, const Value & v);
+std::ostream & printValue(const EvalState & state, std::ostream & str, const Value & v);
+std::string printValue(const EvalState & state, const Value & v);
 
 
 typedef std::pair<std::string, std::string> SearchPathElem;
@@ -77,7 +78,7 @@ public:
 
     static inline std::string derivationNixPath = "//builtin/derivation.nix";
 
-    const Symbol sWith, sOutPath, sDrvPath, sType, sMeta, sName, sValue,
+    const SymbolIdx sWith, sOutPath, sDrvPath, sType, sMeta, sName, sValue,
         sSystem, sOverrides, sOutputs, sOutputName, sIgnoreNulls,
         sFile, sLine, sColumn, sFunctor, sToString,
         sRight, sWrong, sStructuredAttrs, sBuilder, sArgs,
@@ -86,7 +87,7 @@ public:
         sRecurseForDerivations,
         sDescription, sSelf, sEpsilon, sStartSet, sOperator, sKey, sPath,
         sPrefix;
-    Symbol sDerivationNix;
+    SymbolIdx sDerivationNix;
 
     /* If set, force copying files to the Nix store even if they
        already exist there. */
@@ -268,14 +269,14 @@ public:
     [[gnu::noinline, gnu::noreturn]]
     void throwEvalError(const PosIdx pos, const char * s, const std::string & s2, const std::string & s3) const;
     [[gnu::noinline, gnu::noreturn]]
-    void throwEvalError(const PosIdx p1, const char * s, const Symbol & sym, const PosIdx p2) const;
+    void throwEvalError(const PosIdx p1, const char * s, const SymbolIdx sym, const PosIdx p2) const;
     [[gnu::noinline, gnu::noreturn]]
     void throwTypeError(const PosIdx pos, const char * s) const;
     [[gnu::noinline, gnu::noreturn]]
-    void throwTypeError(const PosIdx pos, const char * s, const ExprLambda & fun, const Symbol & s2) const;
+    void throwTypeError(const PosIdx pos, const char * s, const ExprLambda & fun, const SymbolIdx s2) const;
     [[gnu::noinline, gnu::noreturn]]
     void throwTypeError(const PosIdx pos, const Suggestions & suggestions, const char * s,
-        const ExprLambda & fun, const Symbol & s2) const;
+        const ExprLambda & fun, const SymbolIdx s2) const;
     [[gnu::noinline, gnu::noreturn]]
     void throwTypeError(const char * s, const Value & v) const;
     [[gnu::noinline, gnu::noreturn]]
@@ -391,7 +392,7 @@ public:
     inline Value * allocValue();
     inline Env & allocEnv(size_t size);
 
-    Value * allocAttr(Value & vAttrs, const Symbol & name);
+    Value * allocAttr(Value & vAttrs, const SymbolIdx & name);
     Value * allocAttr(Value & vAttrs, std::string_view name);
 
     Bindings * allocBindings(size_t capacity);