Handle integer overflow in for loops

.travis.yml

@@ -11,7 +11,14 @@ matrix:
 #        - INSTALL_LUA="lua=5.2"
 #        - LUA="lua"
     - os: linux
+      addons:
+        apt:
+          sources:
+            - ubuntu-toolchain-r-test
+          packages:
+            - g++-5
       env:
+        - CC="gcc-5"
         - INSTALL_LUA="lua=5.3"
         - LUA="lua"
 #    - os: linux
@@ -49,6 +56,7 @@ matrix:
 #        - LUA="luajit"
 
 before_install:
+  - export CC=$CC
   - if [[ "$TRAVIS_OS_NAME" == "osx" ]]; then brew update; fi
   - pip install hererocks
   - hererocks lua_install -rlatest --$INSTALL_LUA

spec/coder_spec.lua

@@ -11,7 +11,7 @@ local function generate(ast, modname)
     local ok, errmsg = util.set_file_contents(modname .. ".c", generated_code)
     if not ok then return ok, errmsg end
 
-    local CC = "gcc"
+    local CC = os.getenv("CC") or "gcc"
     local CFLAGS = "--std=c99 -O2 -Wall -Ilua/src/ -fPIC"
 
     local cc_cmd = string.format([[
@@ -26,6 +26,16 @@ local function call(modname, code)
     return os.execute(cmd)
 end
 
+local function compile(titan_code, lua_testcase)
+    local ast = parser.parse(titan_code)
+    local ok, err = checker.check(ast, titan_code, "test.titan")
+    assert.truthy(ok, err)
+    ok, err = generate(ast, "titan_test")
+    assert.truthy(ok, err)
+    ok, err = call("titan_test", lua_testcase)
+    return ok, err, ast
+end
+
 describe("Titan code generator", function()
     it("deletes array element", function()
         local code = [[
@@ -122,7 +132,7 @@ describe("Titan code generator", function()
         assert.truthy(ok, err)
     end)
 
-    it("tests integer postive literals in 'for'", function()
+    it("tests integer positive literals in 'for'", function()
         local code = [[
             function forstep(): integer
                 local v: integer = 0
@@ -222,6 +232,24 @@ describe("Titan code generator", function()
         assert.truthy(ok, err)
     end)
 
+    it("avoids integer overflow in 'for' edge case", function()
+        assert.truthy(compile([[
+            function bignumbers(nth: integer):integer
+                local c = 0
+                for i = 0x7FFFFFFFFFFFFFF8,0x7FFFFFFFFFFFFFFF do
+                    c = c + 1
+                    if c == nth then
+                        return i
+                    end
+                end
+                return 0
+            end
+        ]], [[
+            assert(titan_test.bignumbers(8) == 9223372036854775807)
+            assert(titan_test.bignumbers(9) == 0)
+        ]]))
+    end)
+
     it("tests nil element in 'not'", function()
         local code = [[
             function testset(t: {integer}, i: integer, v: integer): integer
@@ -491,7 +519,7 @@ describe("Titan code generator", function()
     it("generates code for array module-local variables", function()
         local code = [[
             local a: {integer} = {}
-            function len(): integer 
+            function len(): integer
                 return #a
             end
             function seta(x: {integer}): nil
@@ -597,7 +625,7 @@ describe("Titan code generator", function()
     it("generates code for exported array variables", function()
         local code = [[
             a: {integer} = {}
-            function len(): integer 
+            function len(): integer
                 return #a
             end
         ]]
@@ -613,7 +641,7 @@ describe("Titan code generator", function()
 
     it("generates code for string length", function()
         local code = [[
-            function len(a: string): integer 
+            function len(a: string): integer
                 return #a
             end
         ]]

titan-compiler/coder.lua

@@ -349,9 +349,11 @@ local function codefor(ctx, node)
     local csstats, csexp = codeexp(ctx, node.start)
     local cfstats, cfexp = codeexp(ctx, node.finish)
     local cinc = ""
+    local covf = ""
     local cvtyp
     if types.equals(node.decl._type, types.Integer) then
         cvtyp = "lua_Integer"
+        covf = "int _foroverflow = 0;"
     else
         cvtyp = "lua_Number"
     end
@@ -382,22 +384,24 @@ local function codefor(ctx, node)
                 local tmpl
                 if types.equals(node.decl._type, types.Integer) then
                     subs.ILIT = c_integer_literal(ilit)
-                    tmpl = "$CVAR = l_castU2S(l_castS2U($CVAR) + $ILIT)"
+                    tmpl = "_foroverflow = __builtin_add_overflow($CVAR, $ILIT, &($CVAR))"
+                    ccmp = render("(!_foroverflow) && ($CVAR <= _forlimit)", subs)
                 else
                     subs.ILIT = c_float_literal(ilit)
                     tmpl = "$CVAR += $ILIT"
+                    ccmp = render("$CVAR <= _forlimit", subs)
                 end
                 cstep = render(tmpl, subs)
-                ccmp = render("$CVAR <= _forlimit", subs)
             else
                 if types.equals(node.decl._type, types.Integer) then
                     subs.NEGILIT = c_integer_literal(-ilit)
-                    cstep = render("$CVAR = l_castU2S(l_castS2U($CVAR) - $NEGILIT)", subs)
+                    cstep = render("_foroverflow = __builtin_sub_overflow($CVAR, $NEGILIT, &($CVAR))", subs)
+                    ccmp = render("(!_foroverflow) && (_forlimit <= $CVAR)", subs)
                 else
                     subs.NEGILIT = c_float_literal(-ilit)
                     cstep = render("$CVAR -= $NEGILIT", subs)
+                    ccmp = render("_forlimit <= $CVAR", subs)
                 end
-                ccmp = render("_forlimit <= $CVAR", subs)
             end
         else
             local cistats, ciexp = codeexp(ctx, node.inc)
@@ -411,20 +415,22 @@ local function codefor(ctx, node)
             })
             local tmpl
             if types.equals(node.decl._type, types.Integer) then
-                tmpl = "$CVAR = l_castU2S(l_castS2U($CVAR) + l_castS2U(_forstep))"
+                tmpl = "_foroverflow = __builtin_add_overflow($CVAR, _forstep, &($CVAR))"
+                ccmp = render("(!_foroverflow) && (0 < _forstep ? ($CVAR <= _forlimit) : (_forlimit <= $CVAR))", subs)
             else
                 tmpl = "$CVAR += _forstep"
+                ccmp = render("0 < _forstep ? ($CVAR <= _forlimit) : (_forlimit <= $CVAR)", subs)
             end
             cstep = render(tmpl, subs)
-            ccmp = render("0 < _forstep ? ($CVAR <= _forlimit) : (_forlimit <= $CVAR)", subs)
         end
     else
         if types.equals(node.decl._type, types.Integer) then
-            cstep = render("$CVAR = l_castU2S(l_castS2U($CVAR) + 1)", subs)
+            cstep = render("_foroverflow = __builtin_add_overflow($CVAR, 1, &($CVAR))", subs)
+            ccmp = render("(!_foroverflow) && ($CVAR <= _forlimit)", subs)
         else
             cstep = render("$CVAR += 1.0", subs)
+            ccmp = render("$CVAR <= _forlimit", subs)
         end
-        ccmp = render("$CVAR <= _forlimit", subs)
     end
     local nallocs = ctx.allocations
     local cblock = codestat(ctx, node.block)
@@ -435,6 +441,7 @@ local function codefor(ctx, node)
             $CSTART
             $CFINISH
             $CINC
+            $COVF
             for($CDECL = _forstart; $CCMP; $CSTEP) {
                 $CBLOCK
                 $CHECKGC
@@ -448,6 +455,7 @@ local function codefor(ctx, node)
         CCMP = ccmp,
         CSTEP = cstep,
         CBLOCK = cblock,
+        COVF = covf,
         CHECKGC = nallocs > 0 and "luaC_checkGC(L);" or ""
     })
 end