.noalias annotation; frontend support (#15419)

* .noalias annotation; frontend support

* added .noalias support to Nim; implements https://github.com/nim-lang/RFCs/issues/204

changelog.md

@@ -283,6 +283,9 @@ proc mydiv(a, b): int {.raises: [].} =
   an experimental feature. In other words, you don't have to write pragma
   `{.experimental: "forLoopMacros".}` if you want to use them.
 
+- Added a ``.noalias`` pragma. It is mapped to C's ``restrict`` keyword for the increased
+  performance this keyword can enable.
+
 
 ## Compiler changes
 

compiler/ast.nim

@@ -229,7 +229,7 @@ type
   TNodeKinds* = set[TNodeKind]
 
 type
-  TSymFlag* = enum    # 43 flags!
+  TSymFlag* = enum    # 46 flags!
     sfUsed,           # read access of sym (for warnings) or simply used
     sfExported,       # symbol is exported from module
     sfFromGeneric,    # symbol is instantiation of a generic; this is needed
@@ -294,6 +294,7 @@ type
                       # or out-of-memory
     sfUsedInFinallyOrExcept  # symbol is used inside an 'except' or 'finally'
     sfSingleUsedTemp  # For temporaries that we know will only be used once
+    sfNoalias         # 'noalias' annotation, means C's 'restrict'
 
   TSymFlags* = set[TSymFlag]
 

compiler/ccgtypes.nim

@@ -474,6 +474,8 @@ proc genProcParams(m: BModule, t: PType, rettype, params: var Rope,
     else:
       params.add(getTypeDescAux(m, param.typ, check, skParam))
     params.add(~" ")
+    if sfNoalias in param.flags:
+      params.add(~"NIM_NOALIAS ")
     params.add(param.loc.r)
     # declare the len field for open arrays:
     var arr = param.typ

compiler/cgen.nim

@@ -503,6 +503,7 @@ proc localVarDecl(p: BProc; n: PNode): Rope =
     #elif skipTypes(s.typ, abstractInst).kind in GcTypeKinds:
     #  decl.add(" GC_GUARD")
     if sfVolatile in s.flags: result.add(" volatile")
+    if sfNoalias in s.flags: result.add(" NIM_NOALIAS")
     result.add(" ")
     result.add(s.loc.r)
   else:
@@ -563,6 +564,7 @@ proc assignGlobalVar(p: BProc, n: PNode; value: Rope) =
         if p.hcrOn: decl.add("*")
         if sfRegister in s.flags: decl.add(" register")
         if sfVolatile in s.flags: decl.add(" volatile")
+        if sfNoalias in s.flags: decl.add(" NIM_NOALIAS")
         if value != nil:
           decl.addf(" $1 = $2;$n", [s.loc.r, value])
         else:
@@ -1261,6 +1263,7 @@ proc genVarPrototype(m: BModule, n: PNode) =
       if lfDynamicLib in sym.loc.flags: m.s[cfsVars].add("*")
       if sfRegister in sym.flags: m.s[cfsVars].add(" register")
       if sfVolatile in sym.flags: m.s[cfsVars].add(" volatile")
+      if sfNoalias in sym.flags: m.s[cfsVars].add(" NIM_NOALIAS")
       m.s[cfsVars].addf(" $1;$n", [sym.loc.r])
       if m.hcrOn: m.initProc.procSec(cpsLocals).addf(
         "\t$1 = ($2*)hcrGetGlobal($3, \"$1\");$n", [sym.loc.r,

compiler/pragmas.nim

@@ -65,14 +65,15 @@ const
     wInheritable, wGensym, wInject, wRequiresInit, wUnchecked, wUnion, wPacked,
     wBorrow, wGcSafe, wPartial, wExplain, wPackage}
   fieldPragmas* = declPragmas + {
-    wGuard, wBitsize, wCursor, wRequiresInit} - {wExportNims, wNodecl} # why exclude these?
+    wGuard, wBitsize, wCursor, wRequiresInit, wNoalias} - {wExportNims, wNodecl} # why exclude these?
   varPragmas* = declPragmas + {wVolatile, wRegister, wThreadVar,
     wMagic, wHeader, wCompilerProc, wCore, wDynlib,
     wNoInit, wCompileTime, wGlobal,
-    wGensym, wInject, wCodegenDecl, wGuard, wGoto, wCursor}
+    wGensym, wInject, wCodegenDecl, wGuard, wGoto, wCursor, wNoalias}
   constPragmas* = declPragmas + {wHeader, wMagic,
     wGensym, wInject,
     wIntDefine, wStrDefine, wBoolDefine, wCompilerProc, wCore}
+  paramPragmas* = {wNoalias, wInject, wGensym}
   letPragmas* = varPragmas
   procTypePragmas* = {FirstCallConv..LastCallConv, wVarargs, wNoSideEffect,
                       wThread, wRaises, wLocks, wTags, wGcSafe,
@@ -864,6 +865,9 @@ proc singlePragma(c: PContext, sym: PSym, n: PNode, i: var int,
       of wRegister:
         noVal(c, it)
         incl(sym.flags, sfRegister)
+      of wNoalias:
+        noVal(c, it)
+        incl(sym.flags, sfNoalias)
       of wThreadVar:
         noVal(c, it)
         incl(sym.flags, {sfThread, sfGlobal})

compiler/semtypes.nim

@@ -1264,6 +1264,8 @@ proc semProcTypeNode(c: PContext, n, genericParams: PNode,
 
     for j in 0..<a.len-2:
       var arg = newSymG(skParam, if a[j].kind == nkPragmaExpr: a[j][0] else: a[j], c)
+      if a[j].kind == nkPragmaExpr:
+        pragma(c, arg, a[j][1], paramPragmas)
       if not hasType and not hasDefault and kind notin {skTemplate, skMacro}:
         let param = strTableGet(c.signatures, arg.name)
         if param != nil: typ = param.typ

compiler/wordrecg.nim

@@ -35,7 +35,7 @@ type
     wColon, wColonColon, wEquals, wDot, wDotDot,
     wStar, wMinus,
     wMagic, wThread, wFinal, wProfiler, wMemTracker, wObjChecks,
-    wIntDefine, wStrDefine, wBoolDefine, wCursor,
+    wIntDefine, wStrDefine, wBoolDefine, wCursor, wNoalias,
 
     wImmediate, wConstructor, wDestructor, wDelegator, wOverride,
     wImportCpp, wImportObjC,
@@ -124,7 +124,7 @@ const
     ":", "::", "=", ".", "..",
     "*", "-",
     "magic", "thread", "final", "profiler", "memtracker", "objchecks",
-    "intdefine", "strdefine", "booldefine", "cursor",
+    "intdefine", "strdefine", "booldefine", "cursor", "noalias",
 
     "immediate", "constructor", "destructor", "delegator", "override",
     "importcpp", "importobjc",

doc/manual_experimental.rst

@@ -1764,6 +1764,21 @@ via ``.noSideEffect``. The rules 3 and 4 can also be approximated by a different
    can only passed to a parameter of a ``.noSideEffect`` proc.
 
 
+Noalias annotation
+==================
+
+Since version 1.4 of the Nim compiler, there is a ``.noalias`` annotation for variables
+and parameters. It is mapped directly to C/C++'s ``restrict`` keyword and means that
+the underlying pointer is pointing to a unique location in memory, no other aliases to
+this location exist. It is *unchecked* that this alias restriction is followed, if the
+restriction is violated, the backend optimizer is free to miscompile the code.
+This is an **unsafe** language feature.
+
+Ideally in later versions of the language, the restriction will be enforced at
+compile time. (Which is also why the name ``noalias`` was choosen instead of a more
+verbose name like ``unsafeAssumeNoAlias``.)
+
+
 Strict funcs
 ============
 

lib/nimbase.h

@@ -588,4 +588,7 @@ NIM_STATIC_ASSERT(sizeof(NI) == sizeof(void*) && NIM_INTBITS == sizeof(NI)*8, ""
   #endif
 #endif
 
+#define NIM_NOALIAS __restrict
+/* __restrict is said to work for all the C(++) compilers out there that we support */
+
 #endif /* NIMBASE_H */

tests/ccgbugs/tnoalias.nim

@@ -0,0 +1,8 @@
+discard """
+  ccodecheck: "\\i@'NIM_CHAR* NIM_NOALIAS x,' @'void* NIM_NOALIAS q'"
+"""
+
+proc p(x {.noalias.}: openArray[char]) =
+  var q {.noalias.}: pointer = unsafeAddr(x[0])
+
+p "abc"