Walker Dwarf

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SummaryDebug information is mostly comparable to GCC.

LLVM is more aggressive at doing function inlining resultingin larger debug tables at O2.

Areas for improvement:Remove duplication of inlined function parameter information.

Enumerators missing in pubnames.System included files in line table if referenced by symbols.Generate stack information for ARM.Generate stack information for AARCH64 function epilogues.

debug_types can provide significant improvements in reducing size ofthe debug information.

No sibling entries reduce debug info sizes at the cost ofloading speed (change can be around 10%).


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What was used in the comparisonComparison performed on a C program of about 70,000 lines

of source in 58 source files and 16 header filesNo library code linked in ... all debug information from thecompiled sourcesUses the same header files

GCC 4.8.2 using:arm-none-eabi-gcc g gpubnames gstrict-dwarfLLVM tip of trunk using:clang target=arm-none-eabi -g


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DWARF debug section sizes

020000

40000

60000

80000

100000

120000

140000160000

180000LLVM -O0LLVM -O1LLVM -O2GCC -O0GCC -O1GCC -O2

Effect of function inlining


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DWARF Tags (symbols)

0

500

1000

1500

2000

2500



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DWARF Attributes (symbol properties)

0

2000

4000

6000

8000

10000

12000LLVM -O0

LLVM -O1LLVM -O2GCC -O0GCC -O1GCC -O2

Due to inlined function arameters

Due to inlined functions


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Are siblings attributes worth the space?LLVM deliberately does not generate entries the

DW_AT_sibling attribute due to the size the occupied in the.debug_info section.GCC does generate DW_AT_sibling entries:

Take ~4% of the .debug_info section size (in this example).

Contribute no actual debug information.Implemented using 4-byte entries.... But of 1508 entries only 96 have a value greater than 255.

What is the effect of not having sibling entries in a debugger?

... Can affect loading speed by about 10%.


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Line Table DifferencesLLVM does not include system header files/directories in the

line table.But symbols defined in the system header files have a file/linereference .... The file entry pointing at the current source file.

LLVM generates End of Function Prologue entries. Needs support in debugger but will help to remove the need forheuristics used by debuggers to find the prologue end.

LLVM (sometimes) marks the function prologue instruction asa place not to put a source level breakpoint.Neither compiler generates Start of Function Epilogueentries.


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Stack Frame DifferencesLLVM does not generate .debug_frame information for the

selected arm-none-eabi target.GCC generates the changes that occur in a functionepilogue, LLVM does not (comparing AARCH64).

The effect of this is that when instruction stepping thefunction epilogue in LLVM generated code, register usage inthe called function may be displayed incorrectly.


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DWARF .debug_types coming soonThe .debug_types section was added in DWARF 4 in order to

help common up duplicated type information. This removalof duplicate information can results in smaller file sizes andas a consequence means debuggers have less debugginginformation to process and load.

I am looking forward to the completion of the currentdevelopment effort to implement support for the .debug_typessection.

GCC already has support for .debug_types, so we can seethe possible benefits of adding this support.


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DWARF Debug section sizes (GCC)

0

50000

100000

150000

200000

250000

GCC -O2GCC -O2 (types)

Main sections that change


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Questions?


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DWARF debug section sizes

0

20000

40000

60000

80000

100000

120000

140000

160000



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DWARF Tags (symbols)

0500

100015002000250030003500400045005000

LLVM -O0LLVM -O1LLVM -O2GCC -O0GCC -O1GCC -O2


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DWARF Attributes (symbol properties)

0

2000

4000

6000

8000

10000

12000

a b s t r a c t

_ o r i g

i n

a c c e s s

i b i l i t y

b y t e

_ s i z e

c a l l_ f i l e

c a l l_ l i n e

c

o m p

_ d i r

c o n s t

_ v a

l u e

d a

t a_

m e m

b e r

_ l o c a

t i o n

d e c l

_ f i l e

d e c l

_ l i n e

d e c l a r a

t i o n

e n c o

d i n g

e n

t r y_ p c

e x t e r n a

l

f r a m

e_

b a s e

h i g h

_ p c

i n l i n e

l a n g u a g e

l o c a

t i o n

l o w

_ p c

n a m e

p r o

d u c e r

p r o

t o t y p e

d

r a n g e s

s i b l i n g

s t m

t_ l i s t

t y p e

u p p e r_

b o u n

d

LLVM -O0LLVM -O1LLVM -O2GCC -O0GCC -O1GCC -O2


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Thank You

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