> > > No, it's been known for a while that the use of macro instructions isn't a > > > good idea in the compiler. It's why we switched the gcc port away from > > > using them as well. > > > > Out of curiosity, when did the switch happen? I thought it was emitting > > macros last time I looked (probably around June) but I could be mistaken. > > > 2001/2002? At one point it emitted things like la, etc. > -eric
In that case it seems much more likely that I misread the cases I'm remembering. Maybe the mnemonic made it look like a macro but the operands used meant it was really a plain instruction. From: Eric Christopher [mailto:[email protected]] Sent: 07 January 2015 17:52 To: Daniel Sanders; [email protected]; Toma Tabacu Cc: [email protected] Subject: Re: [PATCH] [mips] Always clobber $1 for MIPS inline asm. On Wed Jan 07 2015 at 8:40:55 AM Daniel Sanders <[email protected]<mailto:[email protected]>> wrote: > From: Eric Christopher [[email protected]<mailto:[email protected]>] > Sent: 06 January 2015 20:15 > To: Toma Tabacu; Daniel Sanders > Cc: [email protected]<mailto:[email protected]> > Subject: Re: [PATCH] [mips] Always clobber $1 for MIPS inline asm. > > > We could stop using $at but why is that beneficial? Why does universally > > reserving a register to support an (uncommon) construct make more sense to > > you than having that construct properly indicate that it may clobber a > > specific register? The former sounds like a sledgehammer to me. > > Totally is, but blindly adding clobbers is much the same, just a more > focused hammer. I'm not against it, mostly that the original discussion > didn't say any of these things. Agreed. It's currently a compromise between precision and difficulty of implementation. > > The work pre-dates me but as far as I can tell the developers of the Mips > > port deliberately decided not to reserve $at and emit code in '.set noat' > > mode. Presumably this was to see how much of gcc's $at usage is essential > > to the correct operation of the output code. It turns out that > > No, it's been known for a while that the use of macro instructions isn't a > good idea in the compiler. It's why we switched the gcc port away from > using them as well. Out of curiosity, when did the switch happen? I thought it was emitting macros last time I looked (probably around June) but I could be mistaken. 2001/2002? At one point it emitted things like la, etc. -eric > > the answer is 'almost none'. In LLVM's CodeGen, $at isn't particularly > > special. The few instructions/macros/relocations that need a temporary > > register to be $at describe this with an implicit def of AT. Branches like > > B are a good example of this (it's worth pointing out that most of the time > > we don't really need to clobber $at since the branch target will be in > > range). Where the temporary doesn't really need to be $at, we just ask the > > register allocator to give us any register. The hard work to avoid > > reserving $at is already done and I don't see a good reason to change the > > original decision. > > Agreed. Please do make sure that the places where you need to do this are > commented though. It's pretty poorly commented at the moment. > > -eric Toma: Could you add a suitable comment to getClobbers()? > http://reviews.llvm.org/D6638 _______________________________________________ cfe-commits mailing list [email protected]<mailto:[email protected]> http://lists.cs.uiuc.edu/mailman/listinfo/cfe-commits
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