https://gcc.gnu.org/bugzilla/show_bug.cgi?id=108108
Bug ID: 108108
Summary: "gcc -MM" fails to list all dependencies
Product: gcc
Version: 12.1.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: other
Assignee: unassigned at gcc dot gnu.org
Reporter: terra at gnome dot org
Target Milestone: ---
This is a spin-off from bug 52566
#!/bin/sh
rm -f a.h b.h c.c c.o Makefile
(echo '#pragma once'; echo '// Killroy was here') > a.h
cp -p a.h b.h
# or even "ln a.h b.h"
(echo '#include "a.h"'; echo '#include "b.h"') > c.c
gcc -MM c.c > Makefile
echo "------The Makefile:"
cat Makefile
echo "------Compiling first time:"
make CC=gcc c.o
rm b.h
echo '#error "this file was changed"' >b.h
echo "------Compiling second time:"
make CC=gcc c.o
The output I get is
------The Makefile:
c.o: c.c a.h
------Compiling first time:
gcc -c -o c.o c.c
------Compiling second time:
make: 'c.o' is up to date.
Notice that the second compilation does nothing. That's quite wrong. Make has
not been told that c.0 depends on b.h
As seen in bug 52566, this comes down to "#pragma once" deciding that a.h and
b.h are the same file. One could certainly take that point of view for the
purposes of a single compilation. I wouldn't, but fine.
However, "gcc -MM" involves the future. It is not reasonable to assume that
a.h and b.h remain the same file forever. I think it is reasonable to expect
that any quoted #include string will show up as a dependency with no
de-duplication.
Tested on gcc 7.5.0 (opensuse) and 12.1.0 (self-built).
