Improve setjmp()/longjmp() performance.

Exception handling for the C backend used setjmp()/longjmp()
unconditionally. However, on POSIX systems, these functions save and
restore the signal mask, adding considerable overhead to exception
handling, even where no exceptions are involved. The compiler and
library now try to use either _setjmp()/_longjmp() or
sigsetjmp()/siglongjmp() where possible, marked by the defines
"nimRawSetjmp" and "nimSigSetjmp", respectively. The define
"nimStdSetjmp" can be used to revert to setjmp()/longjmp() instead.

compiler/ccgstmts.nim

@@ -832,7 +832,14 @@ proc genTry(p: BProc, t: PNode, d: var TLoc) =
   discard cgsym(p.module, "E_Base")
   linefmt(p, cpsLocals, "#TSafePoint $1;$n", safePoint)
   linefmt(p, cpsStmts, "#pushSafePoint(&$1);$n", safePoint)
-  linefmt(p, cpsStmts, "$1.status = setjmp($1.context);$n", safePoint)
+  if isDefined("nimStdSetjmp"):
+    linefmt(p, cpsStmts, "$1.status = setjmp($1.context);$n", safePoint)
+  elif isDefined("nimSigSetjmp"):
+    linefmt(p, cpsStmts, "$1.status = sigsetjmp($1.context, 0);$n", safePoint)
+  elif isDefined("nimRawSetjmp"):
+    linefmt(p, cpsStmts, "$1.status = _setjmp($1.context);$n", safePoint)
+  else:
+    linefmt(p, cpsStmts, "$1.status = setjmp($1.context);$n", safePoint)
   startBlock(p, "if ($1.status == 0) {$n", [safePoint])
   var length = sonsLen(t)
   add(p.nestedTryStmts, t)

compiler/cgen.nim

@@ -13,7 +13,7 @@ import
   ast, astalgo, strutils, hashes, trees, platform, magicsys, extccomp,
   options, intsets,
   nversion, nimsets, msgs, crc, bitsets, idents, lists, types, ccgutils, os,
-  times, ropes, math, passes, rodread, wordrecg, treetab, cgmeth,
+  times, ropes, math, passes, rodread, wordrecg, treetab, cgmeth, condsyms,
   rodutils, renderer, idgen, cgendata, ccgmerge, semfold, aliases, lowerings,
   semparallel
 

compiler/condsyms.nim

@@ -136,3 +136,5 @@ proc initDefines*() =
   declareSymbol("emulatedthreadvars")
   if platform.OS[targetOS].props.contains(ospLacksThreadVars):
     defineSymbol("emulatedthreadvars")
+  if isDefined("posix"):
+    defineSymbol("nimRawSetjmp")

lib/system/ansi_c.nim

@@ -78,10 +78,23 @@ when defined(macosx):
 else:
   template SIGBUS: expr = SIGSEGV
 
-proc c_longjmp(jmpb: C_JmpBuf, retval: cint) {.
-  header: "<setjmp.h>", importc: "longjmp".}
-proc c_setjmp(jmpb: C_JmpBuf): cint {.
-  header: "<setjmp.h>", importc: "setjmp".}
+when defined(nimSigSetjmp) and not defined(nimStdSetjmp):
+  proc c_longjmp(jmpb: C_JmpBuf, retval: cint) {.
+    header: "<setjmp.h>", importc: "siglongjmp".}
+  template c_setjmp(jmpb: C_JmpBuf): cint =
+    proc c_sigsetjmp(jmpb: C_JmpBuf, savemask: cint): cint {.
+      header: "<setjmp.h>", importc: "sigsetjmp".}
+    c_sigsetjmp(jmpb, 0)
+elif defined(nimRawSetjmp) and not defined(nimStdSetjmp):
+  proc c_longjmp(jmpb: C_JmpBuf, retval: cint) {.
+    header: "<setjmp.h>", importc: "_longjmp".}
+  proc c_setjmp(jmpb: C_JmpBuf): cint {.
+    header: "<setjmp.h>", importc: "_setjmp".}
+else:
+  proc c_longjmp(jmpb: C_JmpBuf, retval: cint) {.
+    header: "<setjmp.h>", importc: "longjmp".}
+  proc c_setjmp(jmpb: C_JmpBuf): cint {.
+    header: "<setjmp.h>", importc: "setjmp".}
 
 proc c_signal(sig: cint, handler: proc (a: cint) {.noconv.}) {.
   importc: "signal", header: "<signal.h>".}