Hi, On Fri, Apr 15, 2011 at 8:39 AM, Paul Römer <roemer.paul@xxxxxxxxxxxxxx> wrote: > Hi, > > I'm playing around with the gcc and its generated assembler code for > some test applications. I recognized that the gcc prefers the use of > the registers eax, (ebx), ecx and edx (x86) for arithmetic > instructions like lea, add, mul, etc. I know that the gcc uses several > register classes to define which registers can be used for which task > (e.g. FIXED_REGS, GENERAL_REGS). > > Is there also a class that explains the described behavior? Or does > the gcc simply looks for unused registers in a special order? > I'll take a stab at this. I would guess that the specialization for the si and di in string moves make them used ... later than other available regs. bp is used as the frame pointer unless -fomit-frame-pointer is used (may be automatically enabled at certain -O levels). > Another question: What is meant by the Q_REGS class? > Based on a comment from gcc/config/i386/i386.h: QImode must go into class Q_REGS. I would guess that Q_REGS are the 8-bit registers. > Thanks, > Paul > kevin
