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* [Bug lto/99559] New: LTO with -Os -ffreestanding -nodefaultlibs may generate call to memcpy, but fail linking even though memcpy is defined
@ 2021-03-12 10:06 kabel at blackhole dot sk
2021-03-12 13:20 ` [Bug lto/99559] " rguenth at gcc dot gnu.org
2021-03-12 18:15 ` pinskia at gcc dot gnu.org
0 siblings, 2 replies; 3+ messages in thread
From: kabel at blackhole dot sk @ 2021-03-12 10:06 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=99559
Bug ID: 99559
Summary: LTO with -Os -ffreestanding -nodefaultlibs may
generate call to memcpy, but fail linking even though
memcpy is defined
Product: gcc
Version: 10.2.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: lto
Assignee: unassigned at gcc dot gnu.org
Reporter: kabel at blackhole dot sk
CC: marxin at gcc dot gnu.org
Target Milestone: ---
Created attachment 50372
--> https://gcc.gnu.org/bugzilla/attachment.cgi?id=50372&action=edit
test.c
When compiling for target where the assignment operator
*dst = *src;
will generate a call to memcpy (for example aarch64), and when also using
compile flags
-ffreestanding -Os -flto
and link flags
-nodefaultlibs -nostartfiles
the linking will fail with
undefined reference to `memcpy'
even if memcpy is defined in the source.
The memcpy is optimized away, even though it is used by the = operator. It
needs to be declared with __attribute__((used)) to not be optimized away.
What is interesting is that if the memcpy function is used explicitly, the
compilation may still fail with -Os, because the code will be inlined.
If memcpy is used explicitly multiple times, it may be generated with symbol
name
memcpy.constprop.0.isra.0
or
memcpy.isra.0
depending on whether the arguments passed are the same or different.
But the linking fill still fail, because the assignment operator will generate
a call to memcpy, and this symbol is not defined, only memcpy.isra.0 or
memcpy.constprop.0.isra.0 is.
I discovered this recently while trying to make U-Boot compile with LTO. We
have solved it by declaring "memcmp", "memset", "memcpy" and "memmove" with
attribute used, since according to GCC man page, calls to this functions may
still be done when -nodefaultlibs is used.
^ permalink raw reply [flat|nested] 3+ messages in thread
* [Bug lto/99559] LTO with -Os -ffreestanding -nodefaultlibs may generate call to memcpy, but fail linking even though memcpy is defined
2021-03-12 10:06 [Bug lto/99559] New: LTO with -Os -ffreestanding -nodefaultlibs may generate call to memcpy, but fail linking even though memcpy is defined kabel at blackhole dot sk
@ 2021-03-12 13:20 ` rguenth at gcc dot gnu.org
2021-03-12 18:15 ` pinskia at gcc dot gnu.org
1 sibling, 0 replies; 3+ messages in thread
From: rguenth at gcc dot gnu.org @ 2021-03-12 13:20 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=99559
Richard Biener <rguenth at gcc dot gnu.org> changed:
What |Removed |Added
----------------------------------------------------------------------------
CC| |hubicka at gcc dot gnu.org,
| |jsm28 at gcc dot gnu.org
--- Comment #1 from Richard Biener <rguenth at gcc dot gnu.org> ---
Yeah, we do remove unused (as in unused from the source) memcpy implementations
early but then introduce uses ourselves.
I suppose we should avoid doing that with -ffreestanding, thus basically add
the used attribute on a select number of definitions automatically with this
option.
I wonder for example whether a static definition of memcpy would need to be
preserved since I think this isn't really special to LTO.
Joseph? Honza?
^ permalink raw reply [flat|nested] 3+ messages in thread
* [Bug lto/99559] LTO with -Os -ffreestanding -nodefaultlibs may generate call to memcpy, but fail linking even though memcpy is defined
2021-03-12 10:06 [Bug lto/99559] New: LTO with -Os -ffreestanding -nodefaultlibs may generate call to memcpy, but fail linking even though memcpy is defined kabel at blackhole dot sk
2021-03-12 13:20 ` [Bug lto/99559] " rguenth at gcc dot gnu.org
@ 2021-03-12 18:15 ` pinskia at gcc dot gnu.org
1 sibling, 0 replies; 3+ messages in thread
From: pinskia at gcc dot gnu.org @ 2021-03-12 18:15 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=99559
Andrew Pinski <pinskia at gcc dot gnu.org> changed:
What |Removed |Added
----------------------------------------------------------------------------
Status|UNCONFIRMED |RESOLVED
Resolution|--- |DUPLICATE
--- Comment #2 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
This is a dup of bug 58203.
*** This bug has been marked as a duplicate of bug 58203 ***
^ permalink raw reply [flat|nested] 3+ messages in thread
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2021-03-12 10:06 [Bug lto/99559] New: LTO with -Os -ffreestanding -nodefaultlibs may generate call to memcpy, but fail linking even though memcpy is defined kabel at blackhole dot sk
2021-03-12 13:20 ` [Bug lto/99559] " rguenth at gcc dot gnu.org
2021-03-12 18:15 ` pinskia at gcc dot gnu.org
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