The attached program yields the expected results when compiled 
with "gcc", "gcc -O", and "gcc -O2":

jik2:/tmp!568$ gcc -O2 test.c
jik2:/tmp!569$ ./a.out
format=Foo %s bar, first arg=baz

However, it yields different results when compiled with "gcc -O3":

jik2:/tmp!570$ gcc -O3 test.c
test.c: In function øformat1ù:
test.c:32: warning: passing argument 3 of ødoprntù makes pointer from integer 
w\
ithout a cast
test.c:32: warning: passing argument 6 of ødoprntù from incompatible pointer 
ty\
pe
test.c: In function ømainù:
test.c:37: warning: passing argument 1 of øformat1ù makes integer from pointer 
\
without a cast
test.c:32: warning: passing argument 3 of ødoprntù makes pointer from integer 
w\
ithout a cast
test.c:32: warning: passing argument 6 of ødoprntù from incompatible pointer 
ty\
pe
jik2:/tmp!571$ ./a.out
format=Foo %s bar, first arg=

I believe this is causing a coredump in Emacs, about which I shall soon file a 
second bug report.

gcc-4.0.2-6, emacs-21.4-10.