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On a 64-bit system, the DWARFDebugLine::Row struct is 32 bytes. Each field has the following byte offsets:

Browse Source 
0-7: Address
8-11: Line
12-13: Column
14-15: File
16-19: Isa
20-23: Discriminator
24+: bit fields

The packing is fine until the "Isa" field, which is an 8-bit int that occupies 4 bytes. We can instead move Discriminator into the 16-19 slot, and pack Isa into the 20-23 range along with the bit fields:

0-7: Address
8-11: Line
12-13: Column
14-15: File
16-19: Discriminator
20-23: Isa + bit fields

This layout is only 24 bytes. This 25% reduction in size may seem small but a large binary can have line tables with thousands of rows stored in a vector.

Patch by Simon Que!

Differential Revision: https://reviews.llvm.org/D27961

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@290931 91177308-0d34-0410-b5e6-96231b3b80d8
This commit is contained in:
Eric Christopher 2017-01-04 02:34:29 +00:00
parent cc80c5c073
commit fdd19a37e7
1 changed files with 3 additions and 3 deletions
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6
include/llvm/DebugInfo/DWARF/DWARFDebugLine.h
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@ -116,12 +116,12 @@ public:
// An unsigned integer indicating the identity of the source file
// corresponding to a machine instruction.
uint16_t File;
// An unsigned integer whose value encodes the applicable instruction set
// architecture for the current instruction.
uint8_t Isa;
// An unsigned integer representing the DWARF path discriminator value
// for this location.
uint32_t Discriminator;
// An unsigned integer whose value encodes the applicable instruction set
// architecture for the current instruction.
uint8_t Isa;
// A boolean indicating that the current instruction is the beginning of a
// statement.
uint8_t IsStmt:1,