Re: Patch: Remove unneeded double operation in libstdc++-v3/src/c++17/fs_path.cc
On Fri, 5 Jan 2024 at 13:00, Martin Küttler
wrote:
>
>
> >>This is a small change to libstdc++ which does not change any behavior.
> >
> > Please CC the libstd...@gcc.gnu.org list on all libstdc++ patches, as
> > documented at https://gcc.gnu.org/lists.html
>
> Acknowledged. Sorry.
>
> >>This change has two, ihmo positive, implications:
> >>
> >> - The implicit conversion from double to int is avoided (Avoiding a
> >> warning).
> >
> > I don't see any warning here. What do you see?
>
> I see "warning: conversion from ‘double’ to ‘int’ may change value
> [-Wfloat-conversion]" This appears to be a specifically enabled warning.
>
> > Looking at path::_List::reserve now, we probably also want to avoid
> > overflow. Although a path with INT_MAX/1.5 components seems
> > implausible for 32-bit and 64-bit targets, it could be a problem for
> > 16-bit targets. I'll take care of that too.
>
> Nice catch.
We also have some redundant code in path::operator/= which can just be
removed, because _List::reserve does it anyway:
if (orig_type == _Type::_Multi)
{
const int curcap = _M_cmpts._M_impl->capacity();
if (capacity > curcap)
capacity = std::max(capacity, (int) (curcap * 1.5));
}
Re: Patch: Remove unneeded double operation in libstdc++-v3/src/c++17/fs_path.cc
>>This is a small change to libstdc++ which does not change any behavior. > > Please CC the libstd...@gcc.gnu.org list on all libstdc++ patches, as > documented at https://gcc.gnu.org/lists.html Acknowledged. Sorry. >>This change has two, ihmo positive, implications: >> >> - The implicit conversion from double to int is avoided (Avoiding a >> warning). > > I don't see any warning here. What do you see? I see "warning: conversion from ‘double’ to ‘int’ may change value [-Wfloat-conversion]" This appears to be a specifically enabled warning. > Looking at path::_List::reserve now, we probably also want to avoid > overflow. Although a path with INT_MAX/1.5 components seems > implausible for 32-bit and 64-bit targets, it could be a problem for > 16-bit targets. I'll take care of that too. Nice catch. Martin -- Kernkonzept GmbH at Dresden, Germany, HRB 31129, CEO Dr.-Ing. Michael Hohmuth
Re: Patch: Remove unneeded double operation in libstdc++-v3/src/c++17/fs_path.cc
On 18/12/23 09:36 +0100, Martin Küttler wrote:
This is a small change to libstdc++ which does not change any behavior.
Please CC the libstd...@gcc.gnu.org list on all libstdc++ patches, as
documented at https://gcc.gnu.org/lists.html
Otherwise I won't see the patches unless I happen to glance at the
gcc-patches archive by chance.
This change has two, ihmo positive, implications:
- The implicit conversion from double to int is avoided (Avoiding a
warning).
I don't see any warning here. What do you see?
- No floating point number is used at all, which could be significant
in some scenarios.
Yes, it seems worth doing for this reason. I'll test+push the patch,
thanks.
Looking at path::_List::reserve now, we probably also want to avoid
overflow. Although a path with INT_MAX/1.5 components seems
implausible for 32-bit and 64-bit targets, it could be a problem for
16-bit targets. I'll take care of that too.
diff --git a/libstdc++-v3/src/c++17/fs_path.cc
b/libstdc++-v3/src/c++17/fs_path.cc
index d65b5482e8b..b47ed0aa7aa 100644
--- a/libstdc++-v3/src/c++17/fs_path.cc
+++ b/libstdc++-v3/src/c++17/fs_path.cc
@@ -447,8 +447,9 @@ path::_List::reserve(int newcap, bool exact = false)
if (curcap < newcap)
{
- if (!exact && newcap < int(1.5 * curcap))
- newcap = 1.5 * curcap;
+ const int nextcap = curcap + curcap / 2;
+ if (!exact && newcap < nextcap)
+ newcap = nextcap;
void* p = ::operator new(sizeof(_Impl) + newcap * sizeof(value_type));
std::unique_ptr<_Impl, _Impl_deleter> newptr(::new(p) _Impl{newcap});
Patch: Remove unneeded double operation in libstdc++-v3/src/c++17/fs_path.cc
This is a small change to libstdc++ which does not change any behavior.
This change has two, ihmo positive, implications:
- The implicit conversion from double to int is avoided (Avoiding a
warning).
- No floating point number is used at all, which could be significant
in some scenarios.
diff --git a/libstdc++-v3/src/c++17/fs_path.cc
b/libstdc++-v3/src/c++17/fs_path.cc
index d65b5482e8b..b47ed0aa7aa 100644
--- a/libstdc++-v3/src/c++17/fs_path.cc
+++ b/libstdc++-v3/src/c++17/fs_path.cc
@@ -447,8 +447,9 @@ path::_List::reserve(int newcap, bool exact = false)
if (curcap < newcap)
{
- if (!exact && newcap < int(1.5 * curcap))
- newcap = 1.5 * curcap;
+ const int nextcap = curcap + curcap / 2;
+ if (!exact && newcap < nextcap)
+ newcap = nextcap;
void* p = ::operator new(sizeof(_Impl) + newcap * sizeof(value_type));
std::unique_ptr<_Impl, _Impl_deleter> newptr(::new(p) _Impl{newcap});
--
Kernkonzept GmbH at Dresden, Germany, HRB 31129, CEO Dr.-Ing. Michael Hohmuth
