On Mon, 18 Apr 2005, Paolo Bonzini wrote: > Roger proposed lowering 64-bit arithmetic to 32-bit in tree-ssa! How > would you do it? Take > > long long a, b, c; > c = a + b; > > Would it be > > c = ((int)a + (int)b) > + ((int) (a >> 32) + (int) (b >> 32) > + ((unsigned int) a < (unsigned int) b)) << 32; > > Or will you introduce new tree codes and uglifying tree-ssa? > Seriously...
I think you may have misinterpreted or over-interpreted what I meant by performing optimizations/lowering, earlier vs. later. When I suggested the i386.c backend should lower DImode operations earlier, my intention was to lower during RTL expansion instead of where it currently happens after reload. Whilst doing it in tree-ssa would technically also be "doing it earlier", this isn't really what I had in mind. The fact that DImode moves aren't lowered, and that the high and low parts of a DImode register can't be independently live/dead (for register allocation) is the reason for PR17236, and why GCC generates poorer code than Intel on something as simple as "a * b". Not that it would be impossible to do this with trees. It's not uncommon for representations to be lowered, for example high-level trees to low-level trees. RTL has always done this, where addressof could be seen as a lowering pass, as can combine, reload, regstack and the process of splitting, where the RTL representation afterwards is a more accurate description of the generated code than the RTL representation before. Roger --
