#52291 - clarify sync_fetch_and_OP for pointers
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Message ID 56A0221C.5060506@gmail.com
State New
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Commit Message

Martin Sebor Jan. 21, 2016, 12:11 a.m. UTC
The bug points out that while the __sync_fetch_and_OP intrinsics are
documented to have semantics equivalent to the "x OP= y" compound
assignment expressions, when used with pointer operands they actually
behave as if they operated on integers.  I.e., they are not scaled by
the size of the pointed-to type.

The attached patch brings the documentation of both the __sync_ and
the __atomic_ intrinsics into alignment with their actual effects.

Martin

PS See also c/64843 for some additional background.

Comments

Jeff Law Jan. 21, 2016, 2:48 a.m. UTC | #1
On 01/20/2016 05:11 PM, Martin Sebor wrote:
> The bug points out that while the __sync_fetch_and_OP intrinsics are
> documented to have semantics equivalent to the "x OP= y" compound
> assignment expressions, when used with pointer operands they actually
> behave as if they operated on integers.  I.e., they are not scaled by
> the size of the pointed-to type.
>
> The attached patch brings the documentation of both the __sync_ and
> the __atomic_ intrinsics into alignment with their actual effects.
>
> Martin
>
> PS See also c/64843 for some additional background.
>
> gcc-52291.patch
>
>
> 2016-01-20  Martin Sebor<msebor@redhat.com>
>
> 	PR c/52291
> 	* extend.texi (__sync Builtins): Clarify the semantics aof
> 	__sync_fetch_and_OP built-ins on pointers.
> 	(__atomic Builtins): Same.
OK
jeff

Patch
diff mbox

2016-01-20  Martin Sebor  <msebor@redhat.com>

	PR c/52291
	* extend.texi (__sync Builtins): Clarify the semantics aof
	__sync_fetch_and_OP built-ins on pointers.
	(__atomic Builtins): Same.
Index: gcc/doc/extend.texi
===================================================================
--- gcc/doc/extend.texi	(revision 232636)
+++ gcc/doc/extend.texi	(working copy)
@@ -9262,8 +9262,11 @@  work on multiple types.
 
 The definition given in the Intel documentation allows only for the use of
 the types @code{int}, @code{long}, @code{long long} or their unsigned
-counterparts.  GCC allows any integral scalar or pointer type that is
-1, 2, 4 or 8 bytes in length.
+counterparts.  GCC allows any scalar type that is 1, 2, 4 or 8 bytes in
+size other than the C type @code{_Bool} or the C++ type @code{bool}.
+Operations on pointer operands are performed as if the operands were
+of the @code{uintptr_t} type.  That is, they are not scaled by the size
+of the type to which the pointer points.
 
 These functions are implemented in terms of the @samp{__atomic}
 builtins (@pxref{__atomic Builtins}).  They should not be used for new
@@ -9309,7 +9312,11 @@  accessible variables should be protected
 @findex __sync_fetch_and_xor
 @findex __sync_fetch_and_nand
 These built-in functions perform the operation suggested by the name, and
-returns the value that had previously been in memory.  That is,
+returns the value that had previously been in memory.  That is, operations
+on integer operands have the following semantics.  Operations on pointer
+operands are performed as if the operands were of the @code{uintptr_t}
+type.  That is, they are not scaled by the size of the type to which
+the pointer points.
 
 @smallexample
 @{ tmp = *ptr; *ptr @var{op}= value; return tmp; @}
@@ -9335,7 +9342,9 @@  as @code{*ptr = ~(tmp & value)} instead
 @findex __sync_xor_and_fetch
 @findex __sync_nand_and_fetch
 These built-in functions perform the operation suggested by the name, and
-return the new value.  That is,
+return the new value.  That is, operations on integer operands have
+the following semantics.  Operations on pointer operands are performed as
+if the operands were of the @code{uintptr_t} type.  That is, they are not
+scaled by the size of the type to which the pointer points.
 
 @smallexample
 @{ *ptr @var{op}= value; return *ptr; @}
@@ -9592,7 +9601,9 @@  pointer.
 @deftypefnx {Built-in Function} @var{type} __atomic_or_fetch (@var{type} *ptr, @var{type} val, int memorder)
 @deftypefnx {Built-in Function} @var{type} __atomic_nand_fetch (@var{type} *ptr, @var{type} val, int memorder)
 These built-in functions perform the operation suggested by the name, and
-return the result of the operation.  That is,
+return the result of the operation.  Operations on pointer operands are
+performed as if the operands were of the @code{uintptr_t} type.  That is,
+they are not scaled by the size of the type to which the pointer points.
 
 @smallexample
 @{ *ptr @var{op}= val; return *ptr; @}
@@ -9610,7 +9621,10 @@  type.  It must not be a Boolean type.  A
 @deftypefnx {Built-in Function} @var{type} __atomic_fetch_or (@var{type} *ptr, @var{type} val, int memorder)
 @deftypefnx {Built-in Function} @var{type} __atomic_fetch_nand (@var{type} *ptr, @var{type} val, int memorder)
 These built-in functions perform the operation suggested by the name, and
-return the value that had previously been in @code{*@var{ptr}}.  That is,
+return the value that had previously been in @code{*@var{ptr}}.  Operations
+on pointer operands are performed as if the operands were of
+the @code{uintptr_t} type.  That is, they are not scaled by the size of
+the type to which the pointer points.
 
 @smallexample
 @{ tmp = *ptr; *ptr @var{op}= val; return tmp; @}