[llvm-dev] Representing X86 long double in Debug Info

[Robinson, Paul via llvm-dev]

Using byte_size=16, bit_size=80 (and optional data_bit_offset) would be the right way to go as far as the DWARF spec is concerned.
Persuading TypeInfo to describe something that would generate this is really the question.
--paulr

From: aprantl at apple.com [mailto:aprantl at apple.com]
Sent: Monday, November 02, 2015 8:39 AM
To: Keno Fischer
Cc: llvm-dev; Duncan P. N. Exon Smith; David Blaikie; Eric Christopher; Robinson, Paul
Subject: Re: Representing X86 long double in Debug Info


On Nov 1, 2015, at 3:14 AM, Keno Fischer <kfischer at college.harvard.edu<mailto:kfischer at college.harvard.edu>> wrote:

I'm in the process of teaching the Verifier to validate that the size of debug info variable and the described value matches (already caught a couple of bugs, both in my frontend and in LLVM itself). However, I've run into the following:

size of passed value (80) does not match size of declared variable (128)
  call void @llvm.dbg.declare(metadata x86_fp80* %x, metadata !11, metadata !13), !dbg !14
  %x = alloca x86_fp80, align 16
!11 = !DILocalVariable(name: "x", scope: !4, file: !1, line: 2, type: !12)
!12 = !DIBasicType(name: "long double", size: 128, align: 128, encoding: DW_ATE_float)
!13 = !DIExpression()

which happens because llvm knows that fp80s are 80bits, while clang declares them as 128bits in the debug info. We might have to special case this in the verifier, but before we do that, I wanted to ask about the following:

Reading the DWARF standard, it seems like the following would be a valid description of an X86 80bit long double:

DW_TAG_base_type
DW_AT_name "long double"
DW_AT_byte_size  16
DW_AT_bit_size      80

As far as I can tell from looking through the source code, both LLDB and GDB would read this just fine, it would be a more accurate description of a long double and if we add support for it in LLVM IR, the verifier would be able to understand what's actually going on. Does this seem like a reasonable thing to have LLVM do or would you prefer to just disable this check in the verifier if there are x86_fp80 types around?

Looking at the code in clang CGDebugInfo just passes through the width of the type as it is described by the TypeInfo, which in turn is defined by the Target. At the moment I do not understand why an x86_fp80 is reported to be 128 bits wide. (Since it’s a type natively supported by LLVM http://llvm.org/docs/LangRef.html#floating-point-types I would have expected it to be more like size=80, align=128)

This looks like a bug to me and the debug info should describe an x86_fp80 as being 80 bits wide, but I don’t understand the mechanics well enough to decide whether the TypeInfo is wrong here or if we should work around it in CGDebugInfo.

[Summoning more people who may have a better idea]

-- adrian
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.llvm.org/pipermail/llvm-dev/attachments/20151102/428d4301/attachment.html>