On February 16, 2018 7:05:10 PM GMT+01:00, Jakub Jelinek <ja...@redhat.com>
wrote:
>On Fri, Feb 16, 2018 at 10:36:27AM -0700, Martin Sebor wrote:
>> The deeper problem is that vec wants to be a POD but GCC
>> instantiates the template on non-POD types. For example
>> genrecog.c instantiates it on struct parameter that has
>> a default ctor.
>>
>> A number of ipa-*.c files instantiate vec on
>ipa_polymorphic_call_context,
>> which also has a default ctor,
>> and ctor even has different semantics than memset (it sets
>> a few members to non-zero). So calling memset here instead
>> of the default ctor could be a potential bug.
>
>Then we should just call memset and then do the placement new for older
>compilers (those certainly didn't have -flifetime-dse=2, so wouldn't
>throw
>away whatever has been there before the ctor), or just fix up
>ipa_polymorphic_call_context so that default ctor clears everything
>rather
>than setting something to true.
>
>In any case, I hope we don't grow too many new uses of value
>initialization
>which is so problematic in older compilers.
>
>Here is what I'll test. AFAIK both the *something = value_init ();
>and ::new (ptr) value_init (); work even in the buggy compilers,
>including
>4.2.[0-3], if value_init has user defined default ctor, just call
>that default ctor and do nothing else, the mishandled cases are only
>for
>classes without those. So it will be enough to make sure we don't
>create
>hash maps of types that need something non-zero in default ctor or put
>such
>types into classes without user provided default ctor.
But the broken compilers will overwrite the memset data with possibly
uninitialized fields of a TARGET_EXPR.
>
>2018-02-16 Jakub Jelinek <ja...@redhat.com>
>
> PR bootstrap/84405
> * vec.h (vec_default_construct): For BROKEN_VALUE_INITIALIZATION use
> memset and value initialization afterwards.
> * hash-table.h (hash_table<Descriptor, Allocator>::empty_slow):
> Likewise.
>
>--- gcc/hash-table.h.jj 2018-02-16 10:04:25.179740963 +0100
>+++ gcc/hash-table.h 2018-02-16 18:48:38.359845266 +0100
>@@ -804,12 +804,11 @@ hash_table<Descriptor, Allocator>::empty
> }
> else
> {
>-#ifndef BROKEN_VALUE_INITIALIZATION
>- for ( ; size; ++entries, --size)
>- *entries = value_type ();
>-#else
>+#ifdef BROKEN_VALUE_INITIALIZATION
> memset (entries, 0, size * sizeof (value_type));
> #endif
>+ for ( ; size; ++entries, --size)
>+ *entries = value_type ();
> }
> m_n_deleted = 0;
> m_n_elements = 0;
>--- gcc/vec.h.jj 2018-02-16 10:04:25.178740963 +0100
>+++ gcc/vec.h 2018-02-16 18:49:21.305850964 +0100
>@@ -490,12 +490,11 @@ template <typename T>
> inline void
> vec_default_construct (T *dst, unsigned n)
> {
>-#ifndef BROKEN_VALUE_INITIALIZATION
>- for ( ; n; ++dst, --n)
>- ::new (static_cast<void*>(dst)) T ();
>-#else
>+#ifdef BROKEN_VALUE_INITIALIZATION
> memset (dst, '\0', sizeof (T) * n);
> #endif
>+ for ( ; n; ++dst, --n)
>+ ::new (static_cast<void*>(dst)) T ();
> }
>
> /* Copy-construct N elements in DST from *SRC. */
>
>
> Jakub
