Microsoft C4244 warnings ("'conversion' conversion from 'type1' to 'type2', possible loss of data")

[Joev Dubach]

I build Lua using Microsoft Visual Studio 2022 17.10.2, and my company uses a BinSkim policy checker that requires me to enable compiler warning C4244 ("'conversion' conversion from 'type1' to 'type2', possible loss of data").

To fix these warnings, any code with an implicit cast that reduces the size of an integral value has to be changed to use an explicit cast. The safety level of the explicit cast is the same as that of the implicit cast. For example, an int value of 257 might be unintentionally replaced by 1 when storing it in an lu_byte, and explicitly casting it to lu_byte before storing it doesn't change this. But I tend to agree that fixing these warnings by making the casts explicit will improve code quality, because when reading the revised code, it's more obvious that "this operation is risky".

Here is a patch against the released Lua 5.4.7 that would fix all these warnings:

>>>>
diff -U 0 lua-5.4.7/src/lapi.c lua-5.4.7-c4244/src/lapi.c
--- lua-5.4.7/src/lapi.c 2024-06-13 18:15:09.000000000 -0400
+++ lua-5.4.7-c4244/src/lapi.c 2024-07-02 17:51:39.076462000 -0400
@@ -1204 +1204 @@
-        g->genminormul = minormul;
+        g->genminormul = (lu_byte)minormul;
@@ -1220 +1220 @@
-        g->gcstepsize = stepsize;
+        g->gcstepsize = (lu_byte)stepsize;
@@ -1279 +1279 @@
-    L->ci->nresults = codeNresults(nresults);  /* mark it */
+    L->ci->nresults = (short)codeNresults(nresults);  /* mark it */
diff -U 0 lua-5.4.7/src/lauxlib.c lua-5.4.7-c4244/src/lauxlib.c
--- lua-5.4.7/src/lauxlib.c 2024-06-13 18:15:09.000000000 -0400
+++ lua-5.4.7-c4244/src/lauxlib.c 2024-07-02 17:51:39.424450100 -0400
@@ -813 +813 @@
-    lf.buff[lf.n++] = c;  /* 'c' is the first character of the stream */
+    lf.buff[lf.n++] = (char)c;  /* 'c' is the first character of the stream */
diff -U 0 lua-5.4.7/src/lcode.c lua-5.4.7-c4244/src/lcode.c
--- lua-5.4.7/src/lcode.c 2024-06-13 18:15:09.000000000 -0400
+++ lua-5.4.7-c4244/src/lcode.c 2024-07-02 17:51:39.611254900 -0400
@@ -342 +342 @@
-  f->lineinfo[pc] = linedif;
+  f->lineinfo[pc] = (ls_byte)linedif;
@@ -482 +482 @@
-  fs->freereg += n;
+  fs->freereg = (lu_byte)(fs->freereg + n);
@@ -1289,2 +1289,2 @@
-    t->u.ind.t = temp;  /* (can't do a direct assignment; values overlap) */
-    t->u.ind.idx = k->u.info;  /* literal short string */
+    t->u.ind.t = (lu_byte)temp;  /* (can't do a direct assignment; values overlap) */
+    t->u.ind.idx = (short)k->u.info;  /* literal short string */
@@ -1295 +1295 @@
-    t->u.ind.t = (t->k == VLOCAL) ? t->u.var.ridx: t->u.info;
+    t->u.ind.t = (lu_byte)((t->k == VLOCAL) ? t->u.var.ridx: t->u.info);
@@ -1297 +1297 @@
-      t->u.ind.idx = k->u.info;  /* literal short string */
+      t->u.ind.idx = (short)k->u.info;  /* literal short string */
@@ -1301 +1301 @@
-      t->u.ind.idx = cast_int(k->u.ival);  /* int. constant in proper range */
+      t->u.ind.idx = (short)cast_int(k->u.ival);  /* int. constant in proper range */
@@ -1305 +1305 @@
-      t->u.ind.idx = luaK_exp2anyreg(fs, k);  /* register */
+      t->u.ind.idx = (short)luaK_exp2anyreg(fs, k);  /* register */
@@ -1822 +1822 @@
-  fs->freereg = base + 1;  /* free registers with list values */
+  fs->freereg = (lu_byte)(base + 1);  /* free registers with list values */
diff -U 0 lua-5.4.7/src/ldebug.c lua-5.4.7-c4244/src/ldebug.c
--- lua-5.4.7/src/ldebug.c 2024-06-13 18:15:09.000000000 -0400
+++ lua-5.4.7-c4244/src/ldebug.c 2024-07-02 17:51:39.800148600 -0400
@@ -921 +921 @@
-  lu_byte mask = L->hookmask;
+  lu_byte mask = (lu_byte)L->hookmask;
diff -U 0 lua-5.4.7/src/ldo.c lua-5.4.7-c4244/src/ldo.c
--- lua-5.4.7/src/ldo.c 2024-06-13 18:15:09.000000000 -0400
+++ lua-5.4.7-c4244/src/ldo.c 2024-07-02 17:51:39.989720700 -0400
@@ -222 +222 @@
-  G(L)->gcstopem = oldgcstop;  /* restore emergency collection */
+  G(L)->gcstopem = (lu_byte)oldgcstop;  /* restore emergency collection */
@@ -342,2 +342,2 @@
-      ci->u2.transferinfo.ftransfer = ftransfer;
-      ci->u2.transferinfo.ntransfer = ntransfer;
+      ci->u2.transferinfo.ftransfer = (unsigned short)ftransfer;
+      ci->u2.transferinfo.ntransfer = (unsigned short)ntransfer;
@@ -506,2 +506,2 @@
-  ci->nresults = nret;
-  ci->callstatus = mask;
+  ci->nresults = (short)nret;
+  ci->callstatus = (unsigned short)mask;
@@ -828 +828 @@
-    setcistrecst(ci, status);  /* status to finish 'pcall' */
+    setcistrecst(ci, (short)status);  /* status to finish 'pcall' */
diff -U 0 lua-5.4.7/src/lgc.c lua-5.4.7-c4244/src/lgc.c
--- lua-5.4.7/src/lgc.c 2024-06-13 18:15:09.000000000 -0400
+++ lua-5.4.7-c4244/src/lgc.c 2024-07-02 17:51:40.256319100 -0400
@@ -263 +263 @@
-  o->tt = tt;
+  o->tt = (lu_byte)tt;
@@ -926 +926 @@
-    g->gcstp = oldgcstp;  /* restore state */
+    g->gcstp = (lu_byte)oldgcstp;  /* restore state */
@@ -1578 +1578 @@
-    g->gcstate = nextstate;
+    g->gcstate = (lu_byte)nextstate;
@@ -1733 +1733 @@
-  g->gcemergency = isemergency;  /* set flag */
+  g->gcemergency = (lu_byte)isemergency;  /* set flag */
diff -U 0 lua-5.4.7/src/liolib.c lua-5.4.7-c4244/src/liolib.c
--- lua-5.4.7/src/liolib.c 2024-06-13 18:15:09.000000000 -0400
+++ lua-5.4.7-c4244/src/liolib.c 2024-07-02 17:51:40.494205800 -0400
@@ -447 +447 @@
-    rn->buff[rn->n++] = rn->c;  /* save current char */
+    rn->buff[rn->n++] = (char)rn->c;  /* save current char */
@@ -531 +531 @@
-      buff[i++] = c;  /* read up to end of line or buffer limit */
+      buff[i++] = (char)c;  /* read up to end of line or buffer limit */
@@ -536 +536 @@
-    luaL_addchar(&b, c);  /* add ending newline to result */
+    luaL_addchar(&b, (char)c);  /* add ending newline to result */
diff -U 0 lua-5.4.7/src/lparser.c lua-5.4.7-c4244/src/lparser.c
--- lua-5.4.7/src/lparser.c 2024-06-13 18:15:09.000000000 -0400
+++ lua-5.4.7-c4244/src/lparser.c 2024-07-02 17:51:40.698073400 -0400
@@ -269 +269 @@
-  e->u.var.vidx = vidx;
+  e->u.var.vidx = (unsigned short)vidx;
@@ -318,2 +318,2 @@
-    var->vd.ridx = reglevel++;
-    var->vd.pidx = registerlocalvar(ls, fs, var->vd.name);
+    var->vd.ridx = (lu_byte)reglevel++;
+    var->vd.pidx = (short)registerlocalvar(ls, fs, var->vd.name);
@@ -500 +500 @@
-    fs->freereg += needed;  /* remove extra values */
+    fs->freereg = (lu_byte)(fs->freereg + needed);  /* remove extra values */
@@ -683 +683 @@
-  fs->freereg = stklevel;  /* free registers */
+  fs->freereg = (lu_byte)stklevel;  /* free registers */
@@ -864 +864 @@
-  fs->freereg = reg;  /* free registers */
+  fs->freereg = (lu_byte)reg;  /* free registers */
@@ -1067,2 +1067,3 @@
-  fs->freereg = base+1;  /* call removes function and arguments and leaves
-                            one result (unless changed later) */
+  fs->freereg = (lu_byte)(base+1);  /* call removes function and
+                                       arguments and leaves one result
+                                       (unless changed later) */
@@ -1341 +1342 @@
-          lh->v.u.ind.t = extra;  /* assignment will use safe copy */
+          lh->v.u.ind.t = (lu_byte)extra;  /* assignment will use safe copy */
@@ -1347 +1348 @@
-          lh->v.u.ind.t = extra;  /* assignment will use safe copy */
+          lh->v.u.ind.t = (lu_byte)extra;  /* assignment will use safe copy */
@@ -1353 +1354 @@
-          lh->v.u.ind.idx = extra;  /* previous assignment will use safe copy */
+          lh->v.u.ind.idx = (short)extra;  /* previous assignment will use safe copy */
@@ -1738 +1739 @@
-    getlocalvardesc(fs, vidx)->vd.kind = kind;
+    getlocalvardesc(fs, vidx)->vd.kind = (lu_byte)kind;
@@ -1913 +1914 @@
-  ls->fs->freereg = luaY_nvarstack(ls->fs);  /* free registers */
+  ls->fs->freereg = (lu_byte)luaY_nvarstack(ls->fs);  /* free registers */
diff -U 0 lua-5.4.7/src/lstring.c lua-5.4.7-c4244/src/lstring.c
--- lua-5.4.7/src/lstring.c 2024-06-13 18:15:09.000000000 -0400
+++ lua-5.4.7-c4244/src/lstring.c 2024-07-02 17:51:40.902147300 -0400
@@ -268 +268 @@
-  u->nuvalue = nuvalue;
+  u->nuvalue = (unsigned short)nuvalue;
diff -U 0 lua-5.4.7/src/lstrlib.c lua-5.4.7-c4244/src/lstrlib.c
--- lua-5.4.7/src/lstrlib.c 2024-06-13 18:15:09.000000000 -0400
+++ lua-5.4.7-c4244/src/lstrlib.c 2024-07-02 17:51:41.657295200 -0400
@@ -131 +131 @@
-    p[i] = tolower(uchar(s[i]));
+    p[i] = (char)tolower(uchar(s[i]));
@@ -144 +144 @@
-    p[i] = toupper(uchar(s[i]));
+    p[i] = (char)toupper(uchar(s[i]));
@@ -541 +541 @@
-  ms->level = level+1;
+  ms->level = (unsigned char)(level+1);
@@ -1019 +1019 @@
-  buff[n] = (d < 10 ? d + '0' : d - 10 + 'a');  /* add to buffer */
+  buff[n] = (char)(d < 10 ? d + '0' : d - 10 + 'a');  /* add to buffer */
@@ -1062 +1062 @@
-      buff[i] = toupper(uchar(buff[i]));
+      buff[i] = (char)toupper(uchar(buff[i]));
<<<<

I can also provide this patch as a file attachment if Google Groups screws up the formatting somehow.

Thanks,

Joev Dubach