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Let's compare the assembler code generated by gcc and gcc4.
$ gcc -S -std=c99 -O3 -o nothing-gcc3.s nothing.c
$ gcc nothing-gcc3.s
nothing-gcc3.s: Assembler messages:
nothing-gcc3.s:8: Error: suffix or operands invalid for `clflush'
$ sed -n '8{p;q}' nothing-gcc3.s
clflush %rdi
That's the instruction it doesn't like.
What does gcc4 do differently?
$ gcc4 -S -std=c99 -O3 -o nothing-gcc4.s nothing.c
$ grep -w clflush nothing-gcc4.s
clflush (%rdi)
Hmm, it just put the register in parentheses... Let's try manually fixing the nothing-gcc3.s assembly code.
$ vim nothing-gcc3.s
$ sed -n '8{p;q}' nothing-gcc3.s
clflush (%rdi)
$ gcc nothing-gcc3.s
Hey, it worked!
Maybe this is a simple addressing-mode bug.
The purpose of compat-gcc-* packages is just compatibility with older RHEL releases, people really should just use gcc unless compatibility is needed.
It makes no sense to fix bugs that aren't going to be fixed in RHEL4, because it really isn't meant as a general purpose compiler.
So, IMHO this is a candidate of changing in RHEL6/RHEL5 only if/after it is released as RHEL4 errata.
Description of problem: On rhel 4 gcc 3.4.x and using the 3.4 compatibility package for rhel5/6 fail on the following: gcc -std=c99 -O3 nothing.c nothing.c: #include <xmmintrin.h> inline void cacheLineInvalidate(const void *start) { _mm_clflush(start); } int main(int argc, char *argv[] ) { return 42; } Output w/gcc3.4: /tmp/ccjdBONN.s: Assembler messages: /tmp/ccjdBONN.s:8: Error: suffix or operands invalid for `clflush' compiles w/gcc4