translate-c: handle negative array indices

Take advantage of wrapping pointer arithmetic to enable negative array
subscripts.

Fixes #8556

Author: ehaas

src/translate_c.zig

@@ -3052,10 +3052,13 @@ fn transArrayAccess(c: *Context, scope: *Scope, stmt: *const clang.ArraySubscrip
     const is_longlong = cIsLongLongInteger(qt);
     const is_signed = cIsSignedInteger(qt);
 
-    const rhs = if (is_longlong or is_signed) blk: {
-        // check if long long first so that signed long long doesn't just become unsigned long long
-        var typeid_node = if (is_longlong) try Tag.identifier.create(c.arena, "usize") else try transQualTypeIntWidthOf(c, qt, false);
-        break :blk try Tag.int_cast.create(c.arena, .{ .lhs = typeid_node, .rhs = try transExpr(c, scope, subscr_expr, .used) });
+    const rhs = if (is_longlong and !is_signed) blk: {
+        var typeid_node = try Tag.identifier.create(c.arena, "usize");
+        const subscr_node = try transExpr(c, scope, subscr_expr, .used);
+        break :blk try Tag.int_cast.create(c.arena, .{ .lhs = typeid_node, .rhs = subscr_node });
+    } else if (is_signed) blk: {
+        const subscr_node = try transExprCoercing(c, scope, subscr_expr, .used);
+        break :blk try usizeCastForWrappingPtrArithmetic(c.arena, subscr_node);
     } else try transExpr(c, scope, subscr_expr, .used);
 
     const node = try Tag.array_access.create(c.arena, .{

test/run_translated_c.zig

@@ -1476,4 +1476,28 @@ pub fn addCases(cases: *tests.RunTranslatedCContext) void {
         \\    return 0;
         \\}
     , "");
+
+    cases.add("Negative array subscript. Issue #8556",
+        \\#include <stdint.h>
+        \\#include <stdlib.h>
+        \\#define TEST_NEGATIVE(type) { type x = -1; if (ptr[x] != 42) abort(); }
+        \\#define TEST_UNSIGNED(type) { type x = 2; if (arr[x] != 42) abort(); }
+        \\int main(void) {
+        \\    int arr[] = {40, 41, 42, 43};
+        \\    int *ptr = arr + 3;
+        \\    if (ptr[-1] != 42) abort();
+        \\    TEST_NEGATIVE(int);
+        \\    TEST_NEGATIVE(long);
+        \\    TEST_NEGATIVE(long long);
+        \\    TEST_NEGATIVE(int64_t);
+        \\    TEST_NEGATIVE(__int128);
+        \\    TEST_UNSIGNED(unsigned);
+        \\    TEST_UNSIGNED(unsigned long);
+        \\    TEST_UNSIGNED(unsigned long long);
+        \\    TEST_UNSIGNED(uint64_t);
+        \\    TEST_UNSIGNED(size_t);
+        \\    TEST_UNSIGNED(unsigned __int128);
+        \\    return 0;
+        \\}
+    , "");
 }

test/translate_c.zig

@@ -2501,7 +2501,7 @@ pub fn addCases(cases: *tests.TranslateCContext) void {
         \\pub export var array: [100]c_int = [1]c_int{0} ** 100;
         \\pub export fn foo(arg_index: c_int) c_int {
         \\    var index = arg_index;
-        \\    return array[@intCast(c_uint, index)];
+        \\    return array[@bitCast(usize, @intCast(isize, index))];
         \\}
         ,
         \\pub const ACCESS = array[@as(c_int, 2)];
@@ -2516,7 +2516,7 @@ pub fn addCases(cases: *tests.TranslateCContext) void {
         \\pub export fn foo() void {
         \\    var a: [10]c_int = undefined;
         \\    var i: c_int = 0;
-        \\    a[@intCast(c_uint, i)] = 0;
+        \\    a[@bitCast(usize, @intCast(isize, i))] = 0;
         \\}
     });
 
@@ -2529,7 +2529,7 @@ pub fn addCases(cases: *tests.TranslateCContext) void {
         \\pub export fn foo() void {
         \\    var a: [10]c_longlong = undefined;
         \\    var i: c_longlong = 0;
-        \\    a[@intCast(usize, i)] = 0;
+        \\    a[@bitCast(usize, @intCast(isize, i))] = 0;
         \\}
     });