Add x86_64-linux-gnu emulator

I wanted a tiny scriptable meltdown proof way to run userspace programs
and visualize how program execution impacts memory. It helps to explain
how things like Actually Portable Executable works. It can show you how
the GCC generated code is going about manipulating matrices and more. I
didn't feel fully comfortable with Qemu and Bochs because I'm not smart
enough to understand them. I wanted something like gVisor but with much
stronger levels of assurances. I wanted a single binary that'll run, on
all major operating systems with an embedded GPL barrier ZIP filesystem
that is tiny enough to transpile to JavaScript and run in browsers too.

https://justine.storage.googleapis.com/emulator625.mp4

tool/build/calculator.c

@@ -18,6 +18,7 @@
 #include "libc/conv/itoa.h"
 #include "libc/dce.h"
 #include "libc/fmt/fmt.h"
+#include "libc/limits.h"
 #include "libc/log/log.h"
 #include "libc/macros.h"
 #include "libc/math.h"
@@ -30,6 +31,7 @@
 #include "libc/sysv/consts/exit.h"
 #include "libc/sysv/consts/sig.h"
 #include "libc/tinymath/emodl.h"
+#include "libc/x/x.h"
 #include "third_party/dtoa/dtoa.h"
 #include "third_party/getopt/getopt.h"
 
@@ -112,6 +114,11 @@ enum Severity {
   kWarning,
 };
 
+enum Exception {
+  kUnderflow = 1,
+  kDivideError,
+};
+
 struct Bytes {
   size_t i, n;
   char *p;
@@ -139,8 +146,8 @@ struct Function {
   const char *doc;
 };
 
+jmp_buf thrower;
 const char *file;
-jmp_buf underflow;
 struct Bytes token;
 struct History history;
 struct Value stack[128];
@@ -200,6 +207,14 @@ void Log(enum Severity l, const char *fmt, ...) {
   if (interactive) ShowStack();
 }
 
+void __on_arithmetic_overflow(void) {
+  Warning("arithmetic overflow");
+}
+
+void OnDivideError(void) {
+  longjmp(thrower, kDivideError);
+}
+
 struct Value Push(struct Value x) {
   if (sp >= ARRAYLEN(stack)) Fatal("stack overflow");
   return (stack[sp++] = x);
@@ -209,7 +224,7 @@ struct Value Pop(void) {
   if (sp) {
     return stack[--sp];
   } else {
-    longjmp(underflow, 1);
+    longjmp(thrower, kUnderflow);
   }
 }
 
@@ -240,10 +255,39 @@ void Pushf(FLOAT f) {
 void OpDrop(void) {
   Pop();
 }
-
 void OpDup(void) {
   Push(Push(Pop()));
 }
+void OpExit(void) {
+  exit(Popi());
+}
+void OpSrand(void) {
+  srand(Popi());
+}
+void OpEmit(void) {
+  fputwc(Popi(), stdout);
+}
+void OpCr(void) {
+  Cr(stdout);
+}
+void OpPrint(void) {
+  printf("%s ", Repr(Pop()));
+}
+void OpComment(void) {
+  comment = true;
+}
+void Glue0f(FLOAT fn(void)) {
+  Pushf(fn());
+}
+void Glue0i(INT fn(void)) {
+  Pushi(fn());
+}
+void Glue1f(FLOAT fn(FLOAT)) {
+  Pushf(fn(Popf()));
+}
+void Glue1i(INT fn(INT)) {
+  Pushi(fn(Popi()));
+}
 
 void OpSwap(void) {
   struct Value a, b;
@@ -262,10 +306,6 @@ void OpOver(void) {
   Push(a);
 }
 
-void OpSrand(void) {
-  srand(Popi());
-}
-
 void OpKey(void) {
   wint_t c;
   ttyraw(kTtyCursor | kTtySigs | kTtyLfToCrLf);
@@ -274,22 +314,6 @@ void OpKey(void) {
   if (c != -1) Pushi(c);
 }
 
-void OpEmit(void) {
-  fputwc(Popi(), stdout);
-}
-
-void OpCr(void) {
-  Cr(stdout);
-}
-
-void OpPrint(void) {
-  printf("%s ", Repr(Pop()));
-}
-
-void OpComment(void) {
-  comment = true;
-}
-
 void OpAssert(void) {
   if (!Popi()) Fatal("assert failed");
 }
@@ -298,30 +322,11 @@ void OpExpect(void) {
   if (!Popi()) Warning("expect failed");
 }
 
-void OpExit(void) {
-  exit(Popi());
-}
-
 void OpMeminfo(void) {
   OpCr();
   OpCr();
-  meminfo(stdout);
-}
-
-void Glue0f(FLOAT fn(void)) {
-  Pushf(fn());
-}
-
-void Glue0i(INT fn(void)) {
-  Pushi(fn());
-}
-
-void Glue1f(FLOAT fn(FLOAT)) {
-  Pushf(fn(Popf()));
-}
-
-void Glue1i(INT fn(INT)) {
-  Pushi(fn(Popi()));
+  fflush(stdout);
+  meminfo(fileno(stdout));
 }
 
 void Glue2f(FLOAT fn(FLOAT, FLOAT)) {
@@ -438,18 +443,25 @@ bool ConsumeLiteral(const char *literal) {
 }
 
 void ConsumeToken(void) {
+  enum Exception ex;
   if (!token.i) return;
   token.p[token.i] = 0;
   token.i = 0;
   if (history.i) history.p[history.i - 1].i = 0;
   if (comment) return;
   if (startswith(token.p, "#!")) return;
-  if (!setjmp(underflow)) {
-    if (CallFunction(token.p)) return;
-    if (ConsumeLiteral(token.p)) return;
-    Warning("bad token: %s", token.p);
-  } else {
-    Warning("stack underflow");
+  switch (setjmp(thrower)) {
+    default:
+      if (CallFunction(token.p)) return;
+      if (ConsumeLiteral(token.p)) return;
+      Warning("bad token: %s", token.p);
+      break;
+    case kUnderflow:
+      Warning("stack underflow");
+      break;
+    case kDivideError:
+      Warning("divide error");
+      break;
   }
 }
 
@@ -549,16 +561,12 @@ void GotoStartOfLine(void) {
 
 void GotoEndOfLine(void) {
 }
-
 void GotoPrevLine(void) {
 }
-
 void GotoNextLine(void) {
 }
-
 void GotoPrevChar(void) {
 }
-
 void GotoNextChar(void) {
 }
 
@@ -694,7 +702,9 @@ void GetOpts(int argc, char *argv[]) {
 
 int main(int argc, char *argv[]) {
   int i, rc;
+  showcrashreports();
   GetOpts(argc, argv);
+  xsigaction(SIGFPE, OnDivideError, 0, 0, 0);
   if (optind == argc) {
     file = "/dev/stdin";
     StartInteractive();