[committed] libstdc++: Add noexcept to std::to_string overloads that don't allocate

[committed] libstdc++: Add noexcept to std::to_string overloads that don't allocate

[Jonathan Wakely]

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When the values is guaranteed to fit in the SSO buffer we know the
string won't allocate, so the function can be noexcept. For 32-bit
integers, we know they need no more than 9 bytes (or 10 with a minus
sign) and the SSO buffer is 15 bytes.

Signed-off-by: Jonathan Wakely <jwakely@redhat.com>

libstdc++-v3/ChangeLog:

	* include/bits/basic_string.h [_GLIBCXX_USE_CXX11_ABI]
	(to_string): Add noexcept if the type width is 32 bits or less.

Tested x86_64-linux. Committed to trunk.


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commit 9d813ddd978aff75001d53fe55ff15e9167bb4d0
Author: Jonathan Wakely <jwakely@redhat.com>
Date:   Wed Sep 15 21:40:20 2021

    libstdc++: Add noexcept to std::to_string overloads that don't allocate
    
    When the values is guaranteed to fit in the SSO buffer we know the
    string won't allocate, so the function can be noexcept. For 32-bit
    integers, we know they need no more than 9 bytes (or 10 with a minus
    sign) and the SSO buffer is 15 bytes.
    
    Signed-off-by: Jonathan Wakely <jwakely@redhat.com>
    
    libstdc++-v3/ChangeLog:
    
            * include/bits/basic_string.h [_GLIBCXX_USE_CXX11_ABI]
            (to_string): Add noexcept if the type width is 32 bits or less.

diff --git a/libstdc++-v3/include/bits/basic_string.h b/libstdc++-v3/include/bits/basic_string.h
index b61fe05efcf..24c454d863a 100644
--- a/libstdc++-v3/include/bits/basic_string.h
+++ b/libstdc++-v3/include/bits/basic_string.h
@@ -3718,6 +3718,9 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
 
   inline string
   to_string(int __val)
+#if _GLIBCXX_USE_CXX11_ABI && (__CHAR_BIT__ * __SIZEOF_INT__) <= 32
+  noexcept // any 32-bit value fits in the SSO buffer
+#endif
   {
     const bool __neg = __val < 0;
     const unsigned __uval = __neg ? (unsigned)~__val + 1u : __val;
@@ -3729,6 +3732,9 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
 
   inline string
   to_string(unsigned __val)
+#if _GLIBCXX_USE_CXX11_ABI && (__CHAR_BIT__ * __SIZEOF_INT__) <= 32
+  noexcept // any 32-bit value fits in the SSO buffer
+#endif
   {
     string __str(__detail::__to_chars_len(__val), '\0');
     __detail::__to_chars_10_impl(&__str[0], __str.size(), __val);
@@ -3737,6 +3743,9 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
 
   inline string
   to_string(long __val)
+#if _GLIBCXX_USE_CXX11_ABI && (__CHAR_BIT__ * __SIZEOF_LONG__) <= 32
+  noexcept // any 32-bit value fits in the SSO buffer
+#endif
   {
     const bool __neg = __val < 0;
     const unsigned long __uval = __neg ? (unsigned long)~__val + 1ul : __val;
@@ -3748,6 +3757,9 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
 
   inline string
   to_string(unsigned long __val)
+#if _GLIBCXX_USE_CXX11_ABI && (__CHAR_BIT__ * __SIZEOF_LONG__) <= 32
+  noexcept // any 32-bit value fits in the SSO buffer
+#endif
   {
     string __str(__detail::__to_chars_len(__val), '\0');
     __detail::__to_chars_10_impl(&__str[0], __str.size(), __val);