<div dir="ltr">Thanks for the lecture :) But I was not planning on changing a single line in LLVM/Clang. I stick to the documentation until I've learned to swim, perhaps even forever. Ah, now I see. You thought I meant "should I modify the code to do this or that." I only meant to change the documentation. Please refer to the patch I've sent on LLVM-commits. That's about what I had in mind. I am fully aware that you cannot simply dive in and hack away on the handling of the %0 temporary. I wouldn't ever dream of doing that!<div>
<br></div><div><br></div><div>-- Mikael<br><div><br></div><div><br></div></div></div><div class="gmail_extra"><br><br><div class="gmail_quote">2013/11/27 Sean Silva <span dir="ltr"><<a href="mailto:chisophugis@gmail.com" target="_blank">chisophugis@gmail.com</a>></span><br>
<blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex"><div dir="ltr"><div>(gah, this turned into a huge digression, sorry)</div><div><br></div>The implicit numbering of BB's seems to be a pretty frequent issue for people. Surprisingly, the issue boils down to simply changing the IR asm (.ll file) syntax so that it can have "unnamed BB's" in a recognizable way that fits in with how unnamed values work (the asmprinter makes an effort to print a comment with the BB number, but the connection is hard to see and it's confusing).<div>
<br></div><div>The thing that makes this not-as-easy-as-it-looks is doing it in a way that preserves compatibility with previous IR (and being able to convince yourself that this is the case), and the fact that the IR-parsing code is a bit twisty (it's not bad, but the way that some things work is subtly different from what you would expect) and you have to find something that "fits well" with what's there, doesn't require major reworking of the existing code, etc.<div>
<br></div><div>An alternative approach is to document very clearly this issue. That might be good in the short term, but IMO the time would be better spent ruminating over a way to fit this into the syntax, and thinking deeply/finding a way to convince yourself and others that this change doesn't break previous .ll files.</div>
<div><br></div><div>It's just about thinking and coming up with a new syntax that fits well and that won't break existing .ll files. The key places for making this round-trip are AssemblyWriter::printBasicBlock in lib/IR/AsmWriter.cpp and LLParser::ParseBasicBlock in lib/AsmParser/LLParser.cpp. The parsing side is likely to be entirely in lib/AsmParser/LLLexer.cpp where you need to find a way to get a new token "LocalLabelID" returned for the new syntax.</div>
<div><br></div><div>To reiterate, the goal of such a change is solely to avoid people getting confused about the implicit numbering. It needs to be reminiscent/suggestive of the instruction numbering syntax to avoid this confusion.</div>
<div><br></div><div>Heck, there may be something within the existing syntax that would work fine for this, but which we can recognize as being "unnamed", rather than a unique name e.g. currently $1: will give the BB a name "$1" (in the sense of getName()), and then "$2:" will give a name "$2", etc., which will cause a lot of pointless string allocations; recognizing a decimal number here might be all that's needed (and updating the outputting code accordingly), although I'm not sure a prefix $ is the best syntax.</div>
<div><br></div><div>Maybe we could even get away with %42: as a BB label and that would be maximally reminiscent. The way that numbered local variables are handled is sort of ad-hoc (it is actually also handled in the Lexer; all the parser sees is lltok::LocalVarID). By just changing LLLexer::LexPercent in LLLexer.cpp to recognize a local label and emit a "LocalLabelID" token, then adding an `else if` to the first `if` in LLParser::ParseBasicBlock, you could probably get a working solution too. However, this introduces an inconsistency in that now there's this pseudo-common syntax (%[0-9]+) for unnamed things for both BB's and instructions, but in the case of instructions, the % sigil is always needed, while the label syntax isn't sigilized by default, but permits this weird sigilized temporary numbered form. Maybe that slight inconsistency is worth it? If the inconsistency is really bothersome, we could also have BB's be able to start sigilized with % in the other case like instructions are (there is no ambiguity because of the trailing `:`), but allow the unsigilized versions for compatibility; this may be more consistent from a semantic perspective too, since we refer to them sigilized when used as instruction operands.</div>
<div><br></div><div>Or maybe you could have the BB be numbered just like `42:` without the sigil. We already lex a label like 42:, but we just have the issue that I mentioned with $1: that we set this string as the getName() value which creates a bunch of useless strings. If you just change the code to emit a "LocalLabelID" for this case and imitate how we handle locally numbered instructions, that could be a pretty clean fix. However, that would change the behavior for how we handle a label like `0:`, for example, with this behavior, the following IR asm would work:</div>
<div><br></div><div><div>define void @foo() {</div><div>0:</div><div> %1 = alloca i8*</div><div> ret void</div><div>}</div><div><br></div><div>but since with our current behavior we handle `0:` as a BB name and set it's getName() as "0", which causes it to not take up the first unnamed value slot (the %0'th one), so then you get an error that %1 should be %0. This may be an annoying forwards-compatibility issue for a while when we still have to work with not-trunk LLVM's, and this incompatibility may not be worth it. Actually all the suggestions that I've made so far have this same issue :/ Actually I think that it is unsolvable without a forwards-compatibility break due to this (any label that was previously accepted would not increment the unnamed local counter, which would cause all the existing unnamed locals to be off by one and cause an error). We do break forward-compatibility from time to time (e.g. the syntax for the new attributes system), so it might not be that big of an issue (although obviously the community will have to decide about the trade-off for a temporary nuisance vs. the issue this solves). If breaking forwards-compatibility is OK, then I would strongly suggest the `0:` syntax or `%0:`.</div>
<div><br></div><div>Hopefully I've given you a bit of the flavor of the issues involved. It's basically just a problem of sitting down and thinking hard, finding something cleanly-implementable that doesn't break backwards compatibility, and checking with the community that the syntax is agreeable and that any forwards-compatibility break is ok.</div>
<div><br></div><div>-- Sean Silva</div></div></div></div><div class="gmail_extra"><br><br><div class="gmail_quote">On Tue, Nov 26, 2013 at 8:02 PM, Mikael Lyngvig <span dir="ltr"><<a href="mailto:mikael@lyngvig.org" target="_blank">mikael@lyngvig.org</a>></span> wrote:<br>
<blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex"><div dir="ltr">The language reference states that local temporaries begin with index 0, but if I try that on my not-entirely-up-to-date v3.4 llc (it is like a week old), I get an error "instruction expected to be numbered '%1'".<div>
<br></div><div>Also, quite a few examples in the LR uses %0 as a local identifier.</div><div><br></div><div>Should I fix those or is it a problem in llc?</div><span><font color="#888888"><div><br></div><div>
<br></div><div>-- Mikael</div><div><br></div>
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