Re: [RFA][PATCH] Isolate erroneous paths optimization

[Ian Lance Taylor <iant at google dot com>]

On Tue, Nov 5, 2013 at 10:50 PM, Marc Glisse <marc.glisse@inria.fr> wrote:
> On Tue, 5 Nov 2013, Ian Lance Taylor wrote:
>
>> This patch actually breaks the Go testsuite.  In Go dereferencing a
>> nil pointer is well-defined: it causes panic that can be caught.  This
>> breaks a test for that functionality by changing the panic to a
>> builtin_trap.
>>
>> That's not a big deal; I'll just disable this optimization in the Go
>> frontend.
>
>
> Shouldn't go use -fno-delete-null-pointer-checks by default then? That
> should disable this optimization and others that rely on the same idea.

No, that is a different optimization with different properties.  The
-fdelete-null-pointer-checks optimization assumes that if you write
    x = *p;
    if (p == NULL) { printf ("NULL\n"); }
that the test p == NULL can not succeed.  In Go, that is true.  If p
is NULL the *p will cause a panic and ordinary code execution will not
proceed.

The recent -fisolate-erroneous-paths optimization will change code
like this:
    if (p == NULL) { printf ("NULL\n"); }
    x = *p;
into code like this:
    if (p == NULL) { printf ("NULL\n"); __builtin_trap (); }
    x = *p;
That is, it will insert a trap rather than dereferencing the pointer
known to be NULL.  This doesn't work for Go because we really do want
the panic, not the __builtin_trap.  This optimization would work fine
for Go if there were a way to explicitly call the panic function
rather than calling trap, but that would be a frontend-dependent
aspect to a middle-end optimization.

Ian