<div dir="ltr">A couple quick comments inline (didn't touch on all points):<div><div class="gmail_extra"><br><div class="gmail_quote">On Wed, Jan 28, 2015 at 1:49 AM, Robin Eklind <span dir="ltr"><<a href="mailto:carl.eklind@myport.ac.uk" target="_blank">carl.eklind@myport.ac.uk</a>></span> wrote:<br><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex">Hello everyone!<br>
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I've recently had a chance to familiarize myself with the nitty-gritty details of LLVM IR. It has been a great learning experience, sometimes frustrating or confusing but mostly rewarding.<br>
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There are a few cases I've come across which seems odd to me. I've tried to cross reference with the language specification and the source code to the best of my abilities, but would like to reach out to an experienced crowd with a few questions.<br>
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Could you help me out by taking a look at these examples? To my novice eyes they seem to highlight inconsistencies in LLVM IR (or the reference implementation), but it is quite likely that I've overlooked something. Please help me out.<br>
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Note: the example source files have been attached and a copy is made available at <a href="https://github.com/mewplay/ll" target="_blank">https://github.com/mewplay/ll</a><br>
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* Item 1 - named pointer types<br>
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It is possible to create a named array pointer type (and many others), but not a named structure pointer type. E.g.<br>
<br>
%x = type [1 x i32]* ; valid.<br>
%x = type {i32}* ; invalid.<br>
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Is this the intended behaviour? Attaching a.ll, b.ll, c.ll and d.ll for reference. All files except d.ll compiles without error using clang version 3.5.1 (tags/RELEASE_351/final).<br>
<br>
> $ clang d.ll<br>
> d.ll:3:16: error: expected top-level entity<br>
> %x = type {i32}*<br>
> ^<br>
> 1 error generated.<br>
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Does it have anything to do with type equality? (just a hunch)<br>
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* Item 2 - equality of named types<br>
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A named integer type is equivalent to its literal type counterpart, but the same is not true for named and literal structures. I am certain that I've read about this before, but can't seem to locate the right section of the language specification; could anyone point me in the right direction? Also, what is the motivation behind this decision? I've skimmed over the code which handles named structure types (in lib/IR/core.cpp), but would love to hear the high level idea.<br>
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Attaching e.ll, f.ll, g.ll and h.ll for reference. All compile just file except h.ll, which produces the following error message (using the same version of clang as above):<br>
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> $ clang h.ll<br>
> h.ll:10:23: error: argument is not of expected type '%x = type { i32 }'<br>
> call void (%x)* @foo({i32} {i32 0})<br>
> ^<br>
> 1 error generated.<br>
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* Item 3 - zero initialized common linkage variables<br>
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According to the language specification common linkage variables are required to have a zero initializer [1]. If so, why are they also required to provide an initial value?<br>
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Attaching i.ll and j.ll for reference. Both compiles just fine and once executed i.ll returns 37 and j.ll return 0. If the common linkage variable @x was not initialized to 0, j.ll would have returned 42.<br>
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* Item 4 - constant common linkage variables<br>
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The language specification states that common linkage variables may not be marked as constant [1]. The parser doesn't seem to enforce this restriction. Would doing so cause any problems?<br>
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Attaching k.ll and l.ll for reference. Both compiles just fine, but once executed k.ll returns 37 (e.g. the constant variable was overwritten) while l.ll segfaults as expected when it tries to overwrite a read-only memory location.<br>
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* Item 5 - appending linkage restrictions<br>
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An extract from the language specification [1]:<br>
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> "appending" linkage may only be applied to global variables of pointer to array type.<br>
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Similarly to item 4 this restriction isn't enforced by the parser. Would it make sense doing so, or is there any problem with such an approach?<br>
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* Item 6 - hash token<br>
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The hash token (#) is defined in lib/AsmParser/LLToken.h (release version 3.5.0 of the LLVM source code) but doesn't seem to be used anywhere else in the source tree. Is this token a historical artefact or does it serve a purpose?<br></blockquote><div><br></div><div>Try deleting it. If the tests pass send a patch. Same for item 7.</div><div> </div><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex">
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* Item 7 - backslash token<br>
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Similarly to item 7 the backslash token doesn't seem to serve a purpose (with regards to release version 3.5.0 of the LLVM source code). Is it used somewhere?<br>
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* Item 8 - quoted labels<br>
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A comment in lib/AsmParser/LLLexer.cpp (once again, release version 3.5.0 of the LLVM source code) describes quoted labels using the following regexp (e.g. at least one character between the double quotes):<br>
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> /// QuoteLabel "[^"]+":<br>
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In contrast the reference implementation accepts quoted labels with zero or more characters between the double quotes. Which is to be trusted? The comment makes more sense as the variable name would effectively be blank otherwise.<br></blockquote><div><br></div><div>Looks an empty name just results in the thing becoming unnamed. That's sort of confusing, but probably not harmful. Maybe we use an empty name as a sentinel for "unnamed", so it sort of just was an accident of the implementation.</div><div> </div><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex">
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* Item 9 - undocumented calling conventions<br>
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The following calling conventions are valid tokens but not described in the language references as of revision 223189:<br>
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intel_ocl_bicc, x86_stdcallcc, x86_fastcallcc, x86_thiscallcc, kw_x86_vectorcallcc, arm_apcscc, arm_aapcscc, arm_aapcs_vfpcc, msp430_intrcc, ptx_kernel, ptx_device, spir_kernel, spir_func, x86_64_sysvcc, x86_64_win64cc, kw_ghccc<br>
<br></blockquote><div><br></div><div>This is just bitrot.</div><div><br></div><div>-- Sean Silva</div><div> </div><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex">
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Lastly I'd just like to thank the LLVM developers for all the time and hard work they've put into this project. I'd especially like to thank you for providing a language specification along side of the reference implementation! Keeping it up to date is a huge task, but also hugely important. Thank you!<br>
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Kind regards<br>
/Robin Eklind<br>
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[1]: <a href="http://llvm.org/docs/LangRef.html#linkage-types" target="_blank">http://llvm.org/docs/LangRef.<u></u>html#linkage-types</a><br>
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