This is the mail archive of the mailing list for the GCC project.

Index Nav: [Date Index] [Subject Index] [Author Index] [Thread Index]
Message Nav: [Date Prev] [Date Next] [Thread Prev] [Thread Next]
Other format: [Raw text]

basic asm and memory clobbers

It seems like a doc update is what is needed to close PR24414 (Old-style asms don't clobber memory). I'm working on this now (phase 1) in the unlikely event that someone is inspired to make a code change here instead.

Like Richard Henderson, I rather expected basic (or "old-style") asm to perform a memory clobber (it doesn't). This bug is now over a decade old, so presumably the question of how this is going to work is settled. IAC, the docs should reflect the current behavior.

Based on this issue plus what I have learned since last updating this page, I'm proposing the attached patch.


CCing the commenters from the bug.
Index: extend.texi
--- extend.texi	(revision 229910)
+++ extend.texi	(working copy)
@@ -7353,7 +7353,8 @@
 @end itemize
 Safely accessing C data and calling functions from basic @code{asm} is more 
-complex than it may appear. To access C data, it is better to use extended 
+complex than it may appear. To access C data (including both local and
+global register variables), use extended
 Do not expect a sequence of @code{asm} statements to remain perfectly 
@@ -7376,6 +7377,12 @@
 visibility of any symbols it references. This may result in GCC discarding 
 those symbols as unreferenced.
+Basic @code{asm} statements are not treated as though they used a "memory"
+clobber, although they do implicitly perform a clobber of the flags
+(@pxref{Clobbers}). Also, there is no implicit clobbering of registers,
+so any registers changed must be restored to their original value before
+exiting the @code{asm}.
 The compiler copies the assembler instructions in a basic @code{asm} 
 verbatim to the assembly language output file, without 
 processing dialects or any of the @samp{%} operators that are available with

Index Nav: [Date Index] [Subject Index] [Author Index] [Thread Index]
Message Nav: [Date Prev] [Date Next] [Thread Prev] [Thread Next]