As of 20100104 (rev. 155613), mainline failsto bootstrap on alpha-dec-osf4.0f
while compiling the stage3 libiberty/regex.o:

% /vol/gcc/obj/gcc-4.5.0-20100104/4.0f-gcc/./prev-gcc/xgcc -B/vol/gcc/obj/gcc-4.5.0-20100104/4.0f-gcc/./prev-gcc/ -B/vol/gcc/alpha-dec-osf4.0f/bin/ -B/vol/gcc/alpha-dec-osf4.0f/bin/ -B/vol/gcc/alpha-dec-osf4.0f/lib/ -isystem /vol/gcc/alpha-dec-osf4.0f/include -isystem /vol/gcc/alpha-dec-osf4.0f/sys-include    -c -DHAVE_CONFIG_H -g -O2  -I. -I/vol/gcc/src/gcc-dist/libiberty/../include  -W -Wall -Wwrite-strings -Wc++-compat -Wstrict-prototypes -pedantic  /vol/gcc/src/gcc-dist/libiberty/regex.c -o regex.o -save-temps -v
Reading specs from /vol/gcc/obj/gcc-4.5.0-20100104/4.0f-gcc/./prev-gcc/specs
COLLECT_GCC=/vol/gcc/obj/gcc-4.5.0-20100104/4.0f-gcc/./prev-gcc/xgcc
COLLECT_LTO_WRAPPER=/vol/gcc/obj/gcc-4.5.0-20100104/4.0f-gcc/./prev-gcc/lto-wrapper
Target: alpha-dec-osf4.0f
Configured with: /vol/gcc/src/gcc-dist/configure --prefix=/vol/gcc --with-local-prefix=/vol/gcc --disable-nls --with-gmp=/vol/gcc --with-mpfr=/vol/gcc --with-mpc=/vol/gcc --with-libelf=/vol/gcc --host alpha-dec-osf4.0f --build alpha-dec-osf4.0f --target alpha-dec-osf4.0f --enable-languages=c,c++,fortran,java,objc
Thread model: posix
gcc version 4.5.0 20100104 (experimental) [trunk revision 155613] (GCC) 
COLLECT_GCC_OPTIONS='-B/vol/gcc/obj/gcc-4.5.0-20100104/4.0f-gcc/./prev-gcc/' '-B/vol/gcc/alpha-dec-osf4.0f/bin/' '-B/vol/gcc/alpha-dec-osf4.0f/bin/' '-B/vol/gcc/alpha-dec-osf4.0f/lib/' '-isystem' '/vol/gcc/alpha-dec-osf4.0f/include' '-isystem' '/vol/gcc/alpha-dec-osf4.0f/sys-include' '-c' '-DHAVE_CONFIG_H' '-g' '-O2' '-I.' '-I/vol/gcc/src/gcc-dist/libiberty/../include' '-W' '-Wall' '-Wwrite-strings' '-Wc++-compat' '-Wstrict-prototypes' '-pedantic' '-o' 'regex.o' '-save-temps' '-v'
 /vol/gcc/obj/gcc-4.5.0-20100104/4.0f-gcc/./prev-gcc/cc1 -E -quiet -v -I. -I/vol/gcc/src/gcc-dist/libiberty/../include -iprefix /tmp_mnt/vol/gcc/obj/gcc-4.5.0-20100104/4.0f-gcc/prev-gcc/../lib/gcc/alpha-dec-osf4.0f/4.5.0/ -isystem /vol/gcc/obj/gcc-4.5.0-20100104/4.0f-gcc/./prev-gcc/include -isystem /vol/gcc/obj/gcc-4.5.0-20100104/4.0f-gcc/./prev-gcc/include-fixed -DHAVE_CONFIG_H -isystem /vol/gcc/alpha-dec-osf4.0f/include -isystem /vol/gcc/alpha-dec-osf4.0f/sys-include /vol/gcc/src/gcc-dist/libiberty/regex.c -W -Wall -Wwrite-strings -Wc++-compat -Wstrict-prototypes -pedantic -g -fworking-directory -O2 -fpch-preprocess -o regex.i
[...]
 /vol/gcc/obj/gcc-4.5.0-20100104/4.0f-gcc/./prev-gcc/cc1 -fpreprocessed regex.i -quiet -dumpbase regex.c -auxbase-strip regex.o -g -O2 -W -Wall -Wwrite-strings -Wc++-compat -Wstrict-prototypes -pedantic -version -o regex.s
GNU C (GCC) version 4.5.0 20100104 (experimental) [trunk revision 155613] (alpha-dec-osf4.0f)
        compiled by GNU C version 4.5.0 20100104 (experimental) [trunk revision 155613], GMP version 4.2.1, MPFR version 2.3.2, MPC version 0.8
GGC heuristics: --param ggc-min-expand=30 --param ggc-min-heapsize=4096
GNU C (GCC) version 4.5.0 20100104 (experimental) [trunk revision 155613] (alpha-dec-osf4.0f)
        compiled by GNU C version 4.5.0 20100104 (experimental) [trunk revision 155613], GMP version 4.2.1, MPFR version 2.3.2, MPC version 0.8
GGC heuristics: --param ggc-min-expand=30 --param ggc-min-heapsize=4096
Compiler executable checksum: 8f3075b99b6a6af3bebe7ab0e925e5fb
In file included from /vol/gcc/src/gcc-dist/libiberty/regex.c:638:0:
/vol/gcc/src/gcc-dist/libiberty/regex.c: In function 'byte_re_compile_fastmap':
/vol/gcc/src/gcc-dist/libiberty/regex.c:4660:20: warning: '({anonymous})' may be used uninitialized in this function
/vol/gcc/src/gcc-dist/libiberty/regex.c:4653:22: note: '({anonymous})' was declared here
In file included from /vol/gcc/src/gcc-dist/libiberty/regex.c:638:0:
/vol/gcc/src/gcc-dist/libiberty/regex.c: In function 'byte_re_match_2_internal':
/vol/gcc/src/gcc-dist/libiberty/regex.c:5543:1: error: definition in block 8 does not dominate use in block 7
for SSA_NAME: D.6360_246 in statement:
D.6361_247 = regstart_61 + D.6360_246;
/vol/gcc/src/gcc-dist/libiberty/regex.c:5543:1: internal compiler error: verify_ssa failed

The problem doesn't happen at -O0, only with -O and above:

% cc1 -fpreprocessed regex.i -quiet -g -O
In file included from /vol/gcc/src/gcc-dist/libiberty/regex.c:638:0:
/vol/gcc/src/gcc-dist/libiberty/regex.c: In function 'byte_re_match_2_internal':
/vol/gcc/src/gcc-dist/libiberty/regex.c:5543:1: error: definition in block 13 does not dominate use in block 12
for SSA_NAME: D.6360_246 in statement:
D.6361_247 = regstart_61 + D.6360_246;
/vol/gcc/src/gcc-dist/libiberty/regex.c:5543:1: internal compiler error: verify_ssa failed