Re: [PATCH] Improve dead code elimination with -fsanitize=address (PR84307)

[Richard Biener <richard.guenther@gmail.com>]

On February 9, 2018 6:23:37 PM GMT+01:00, Paolo Bonzini <bonzini@gnu.org> wrote:
>On 09/02/2018 17:40, Richard Biener wrote:
>> On February 9, 2018 5:08:24 PM GMT+01:00, Paolo Bonzini
><bonzini@gnu.org> wrote:
>>> Another possibility which I considered but did not implement is to
>mark
>>> the UNPOISON calls so that they do not cause the parameter to
>escape.
>> 
>> I'd do this, thus assign proper fnspec attributes to the asan
>functions. 
>
>Hmm, actually that might be as simple as fixing a typo:
>
>diff --git a/gcc/internal-fn.def b/gcc/internal-fn.def
>index 5970d0e..15d6151 100644
>--- a/gcc/internal-fn.def
>+++ b/gcc/internal-fn.def
>@@ -255,8 +255,8 @@ DEF_INTERNAL_FN (UBSAN_PTR, ECF_LEAF | ECF_NOTHROW,
>".R.")
> DEF_INTERNAL_FN (UBSAN_OBJECT_SIZE, ECF_LEAF | ECF_NOTHROW, NULL)
> DEF_INTERNAL_FN (ABNORMAL_DISPATCHER, ECF_NORETURN, NULL)
>DEF_INTERNAL_FN (BUILTIN_EXPECT, ECF_CONST | ECF_LEAF | ECF_NOTHROW,
>NULL)
>-DEF_INTERNAL_FN (ASAN_CHECK, ECF_TM_PURE | ECF_LEAF | ECF_NOTHROW,
>".R...")
>-DEF_INTERNAL_FN (ASAN_MARK, ECF_LEAF | ECF_NOTHROW, ".R..")
>+DEF_INTERNAL_FN (ASAN_CHECK, ECF_TM_PURE | ECF_LEAF | ECF_NOTHROW,
>"..R..")
>+DEF_INTERNAL_FN (ASAN_MARK, ECF_LEAF | ECF_NOTHROW, "..W.")
>DEF_INTERNAL_FN (ASAN_POISON, ECF_LEAF | ECF_NOTHROW | ECF_NOVOPS,
>NULL)
>DEF_INTERNAL_FN (ASAN_POISON_USE, ECF_LEAF | ECF_NOTHROW | ECF_NOVOPS,
>NULL)
>DEF_INTERNAL_FN (ADD_OVERFLOW, ECF_CONST | ECF_LEAF | ECF_NOTHROW,
>NULL)
>
>which indeed fixes the testcase and seems not to break asan.exp.

Huh. Need to double check why that makes sense ;) 

>'W' is needed to avoid breaking the pr78541.c testcase, and I think it
>makes sense since ASAN_MARK is "writing" the state of the object
>(in the test case FRE moves a dereference across a poisoning).
>
>I'll look at it next week.  Someone maybe should take a look at ubsan
>fnspecs too.
>
>Paolo