Re: [PATCH] Set default to -fomit-frame-pointer

[Wilco Dijkstra <Wilco dot Dijkstra at arm dot com>]

Sandra Loosemore wrote:

> I'd prefer that you remove the reference to configure options entirely 
> here.  Nowadays most GCC users install a package provided by their OS 
> distribution, Linaro, etc, rather than trying to build GCC from scratch.

OK, I've removed that reference. Similarly the FRAME_POINTER_REQUIRED
bit as that statement is not only irrelevant but also completely incorrect.

> > +Enabled at levels @option{-O}, @option{-O1}, @option{-O2}, @option{-O3},
> > +@option{-Os} and @option{-Og}.
>
> This last sentence makes no sense.  If the option is now enabled by 
> default, then the optimization level is irrelevant.

It's enabled from -O onwards, so I've changed it to the standard form used
elsewhere and updated the table for -O:

+Enabled by default at @option{-O} and higher.

Here is the cleaned up and simplified version:


diff --git a/gcc/doc/invoke.texi b/gcc/doc/invoke.texi
index 2ef88e081f982f5619132cc33ce23c3fb542ae11..158c9ae3f1297a1265fc974cd3e6825d8f5be096 100644
--- a/gcc/doc/invoke.texi
+++ b/gcc/doc/invoke.texi
@@ -7258,6 +7258,7 @@ compilation time.
 -fipa-reference @gol
 -fmerge-constants @gol
 -fmove-loop-invariants @gol
+-fomit-frame-pointer @gol
 -freorder-blocks @gol
 -fshrink-wrap @gol
 -fshrink-wrap-separate @gol
@@ -7282,9 +7283,6 @@ compilation time.
 -ftree-ter @gol
 -funit-at-a-time}
 
-@option{-O} also turns on @option{-fomit-frame-pointer} on machines
-where doing so does not interfere with debugging.
-
 @item -O2
 @opindex O2
 Optimize even more.  GCC performs nearly all supported optimizations
@@ -7436,29 +7434,18 @@ The default is @option{-ffp-contract=fast}.
 
 @item -fomit-frame-pointer
 @opindex fomit-frame-pointer
-Don't keep the frame pointer in a register for functions that
-don't need one.  This avoids the instructions to save, set up and
-restore frame pointers; it also makes an extra register available
-in many functions.  @strong{It also makes debugging impossible on
-some machines.}
-
-On some machines, such as the VAX, this flag has no effect, because
-the standard calling sequence automatically handles the frame pointer
-and nothing is saved by pretending it doesn't exist.  The
-machine-description macro @code{FRAME_POINTER_REQUIRED} controls
-whether a target machine supports this flag.  @xref{Registers,,Register
-Usage, gccint, GNU Compiler Collection (GCC) Internals}.
-
-The default setting (when not optimizing for
-size) for 32-bit GNU/Linux x86 and 32-bit Darwin x86 targets is
-@option{-fomit-frame-pointer}.  You can configure GCC with the
-@option{--enable-frame-pointer} configure option to change the default.
-
-Note that @option{-fno-omit-frame-pointer} doesn't force a new stack
-frame for all functions if it isn't otherwise needed, and hence doesn't
-guarantee a new frame pointer for all functions.
+Omit the frame pointer in functions that don't need one.  This avoids the
+instructions to save, set up and restore the frame pointer; on many targets
+it also makes an extra register available.
 
-Enabled at levels @option{-O}, @option{-O2}, @option{-O3}, @option{-Os}.
+On some targets this flag has no effect because the standard calling sequence
+always uses a frame pointer, so it cannot be omitted.
+
+Note that @option{-fno-omit-frame-pointer} doesn't guarantee the frame pointer
+is used in all functions.  Several targets always omit the frame pointer in
+leaf functions.
+
+Enabled by default at @option{-O} and higher.
 
 @item -foptimize-sibling-calls
 @opindex foptimize-sibling-calls
@@ -16753,9 +16740,7 @@ Certain other options, such as @option{-mid-shared-library} and
 @opindex momit-leaf-frame-pointer
 Don't keep the frame pointer in a register for leaf functions.  This
 avoids the instructions to save, set up and restore frame pointers and
-makes an extra register available in leaf functions.  The option
-@option{-fomit-frame-pointer} removes the frame pointer for all functions,
-which might make debugging harder.
+makes an extra register available in leaf functions.
 
 @item -mspecld-anomaly
 @opindex mspecld-anomaly