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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.
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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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