compiler thoughts

Friday April 21, 2006

gcc flag compatibility

As you know gcc4ss accepts all the flags that plain gcc has. In the first release we mapped -mcpu flags to their -xarch equivalents, but didn't do any backwards mapping from studio flags (-xarch, -xtarget, -xchip) to gcc flags (-mcpu,-mtune) and it caused some confusion. Some users noticed -mcpu=v7 passed to 'cc1' component and made a conclusion that gcc4ss generates v7 code. That's not the case. Sun IR generation in cc1 is the same for any -mcpu flag and actual architecture and tuning is controlled by backend flags passed to 'cg' component.

For the next gcc4ss update the flag mapping -x* <-> -m* is going to be more consistent. The draft for new mapping is here: http://cooltools.sunsource.net/gcc/mcpu.html

The default architecture is still v8plus (for 32-bit) and v9 (for 64-bit).
In GCC terminology v8plus architecture is -mcpu=v9.
Don't confuse it with gcc's -mv8plus flag. v8plus mode in gcc is actually default on solaris, but it gets overwritten by -mcpu=v7 default. So gcc 4.0 and 4.1 default on solaris is v7 isa. Confusing right? gcc 4.2 is switching to -mcpu=v9 for solaris 7+, so it's going to generate v8plus isa by default.
For gcc4ss -mv8plus is going to work as its name would intend. -mv8plus in gcc4ss will turn v8plus isa on, whereas in plain gcc 4.x -mv8plus makes no difference at all. GCC docs don't help here. Docs quote:
"With -mv8plus, GCC generates code for the SPARC-V8+ ABI. The difference from the V8 ABI is that the global and out registers are considered 64-bit wide. This is enabled by default on Solaris in 32-bit mode for all SPARC-V9 processors."
So it's natural to expect that and I've seen quite a few gcc users adding -mv8plus hoping to get v8plus isa and disappointed that their code didn't improve. The problem is that the gcc driver is passing -mcpu=v7 to cc1 and it overwrites -mv8plus flag and v8plus default. Even if gcc driver didn't pass it, cc1 internal default of -mcpu=v7 is stronger than -mv8plus flag, so the users don't see any v8plus code unless they specify -mcpu=v9. At least -mno-v8plus is working as expected.
gcc4ss's -mv8plus is going to really change isa to v8plus.