Created attachment 690281 [details]
Preprocessed testcase

Description of problem:
Inline assembler in attached testcase (which is a stripped down code section from the vbox kmod) fails to compile with gcc 4.8.

Version-Release number of selected component (if applicable):
gcc-4.8.0-0.6.fc19.x86_64

How reproducible:
Always

Steps to Reproduce:
1. grab attached testcase
2. gcc -Wall -O2 -c -o test.o test.i
  
Actual results:
$ gcc -Wall -O2 -c -o test.o test.i
test.c: In function ‘test’:
test.c:14:9: error: ‘asm’ operand has impossible constraints
         __asm__ __volatile__("repe; scasl\n\t"

Additional info:
- code trimmed for 64bit CPU
- error triggers only with optimization enabled (-O1, -O2, -O3, -Os, -Ofast)
- testcase works with gcc version 4.7.2 20121109 (Red Hat 4.7.2-9) (GCC)