On Oct 22, 2007, at 9:35 AM, Chris Lattner wrote: > On Oct 20, 2007, at 11:49 AM, Dale Johannesen wrote: >> On Oct 20, 2007, at 10:29 AM, Chris Lattner wrote: >>> This is just a really ugly compatibility hack. Apparently GCC >>> doesn't do cross jumping of inline asm statements as much as we do, >>> and we've hit a significant body of code that depends on this not >>> happening. >>> >>> The code in question is clearly broken, but at this point, I think >>> it's important for LLVM to be "bug compatible" with GCC if it >>> doesn't >>> cost much. In this case, we only lose tail merging of inline asm >>> statements, which impacts code size but not performance. >> >> Bill's change is not clearly the wrong thing. People using asm's are >> generally trying to do something they can't do more >> straightforwardly; they know what code they want the compiler to >> produce, and regard writing asm's as an exercise in coercing the >> compiler into producing it. (They should be writing in assembler >> IMO, but gcc decided years ago to support asm, and we're all stuck >> with it now.) Before moving to llvm, I was maintaining gcc for some >> time; I never had anybody complain their asm's were not optimized >> enough, and I got many complaints that asm's were optimized too much. > > I'm sympathetic to that viewpoint, but it can't be made to work. > > In this particular case, the user is using pairs of asms and > expecting them to come out in the .s file in the same order as they > wrote. There is no way the compiler can reasonably guarantee this to > work: it would prevent all block reordering and control flow > optimizations etc.
Yeah, turning all that off is a bit much. (I did, however, get the question "how do I prevent the compiler from rearranging my code" more than once from the asm users. The current answer is "you don't", but this is something that certain people want.) _______________________________________________ llvm-commits mailing list llvm-commits@cs.uiuc.edu http://lists.cs.uiuc.edu/mailman/listinfo/llvm-commits
