compiler-rt: remove dead code

`__addosi4`, `__addodi4`, `__addoti4`, `__subosi4`, `__subodi4`, and
`__suboti4` were all functions which we invented for no apparent reason.
Neither LLVM, nor GCC, nor the Zig compiler use these functions. It
appears the functions were created in a kind of misunderstanding of an
old language proposal; see #10824.

There is no benefit to these functions existing; if a Zig compiler
backend needs this operation, it is trivial to implement, and *far*
simpler than calling a compiler-rt routine. Therefore, this commit
deletes them. A small amount of that code was used by other parts of
compiler-rt; the logic is trivial so has just been inlined where needed.
I also chose to quickly implement `__addvdi3` (a standard function)
because it is trivial and we already implement the `sub` parallel.

Author: mlugg

lib/compiler_rt.zig

@@ -28,12 +28,13 @@ comptime {
     _ = @import("compiler_rt/negv.zig");
 
     _ = @import("compiler_rt/addvsi3.zig");
+    _ = @import("compiler_rt/addvdi3.zig");
+
     _ = @import("compiler_rt/subvsi3.zig");
     _ = @import("compiler_rt/subvdi3.zig");
+
     _ = @import("compiler_rt/mulvsi3.zig");
 
-    _ = @import("compiler_rt/addo.zig");
-    _ = @import("compiler_rt/subo.zig");
     _ = @import("compiler_rt/mulo.zig");
 
     // Float routines

lib/compiler_rt/addo.zig

@@ -0,0 +1,26 @@
+const common = @import("./common.zig");
+const testing = @import("std").testing;
+
+pub const panic = common.panic;
+
+comptime {
+    @export(&__addvdi3, .{ .name = "__addvdi3", .linkage = common.linkage, .visibility = common.visibility });
+}
+
+pub fn __addvdi3(a: i64, b: i64) callconv(.c) i64 {
+    const sum = a +% b;
+    // Overflow occurred iff both operands have the same sign, and the sign of the sum does
+    // not match it. In other words, iff the sum sign is not the sign of either operand.
+    if (((sum ^ a) & (sum ^ b)) < 0) @panic("compiler-rt: integer overflow");
+    return sum;
+}
+
+test "addvdi3" {
+    // const min: i64 = -9223372036854775808
+    // const max: i64 = 9223372036854775807
+    // TODO write panic handler for testing panics
+    // try test__addvdi3(-9223372036854775808, -1, -1); // panic
+    // try test__addvdi3(9223372036854775807, 1, 1);  // panic
+    try testing.expectEqual(-9223372036854775808, __addvdi3(-9223372036854775807, -1));
+    try testing.expectEqual(9223372036854775807, __addvdi3(9223372036854775806, 1));
+}

lib/compiler_rt/addodi4_test.zig

@@ -1,4 +1,3 @@
-const addv = @import("addo.zig");
 const common = @import("./common.zig");
 const testing = @import("std").testing;
 
@@ -9,9 +8,10 @@ comptime {
 }
 
 pub fn __addvsi3(a: i32, b: i32) callconv(.c) i32 {
-    var overflow: c_int = 0;
-    const sum = addv.__addosi4(a, b, &overflow);
-    if (overflow != 0) @panic("compiler-rt: integer overflow");
+    const sum = a +% b;
+    // Overflow occurred iff both operands have the same sign, and the sign of the sum does
+    // not match it. In other words, iff the sum sign is not the sign of either operand.
+    if (((sum ^ a) & (sum ^ b)) < 0) @panic("compiler-rt: integer overflow");
     return sum;
 }