||Line Number Table to Support Profiling
||Accepted with modifications
PC sampling is often used to profile the execution paths of an
application, and the sampled data needs to be fed back to the compiler
for profile-based opimization. In order to map the individual PC
samples to the original source code, the optimizer needs to use the
DWARF line number table. This approach works in general, but it is
currently unable to distinguish among multiple paths of execution on a
single source line.
In order to distinguish these additional paths of execution, we
propose to add an additional column to the DWARF line number table,
which will hold a "discriminator", a simple integer to discriminate
between the n different execution paths associated with a given line
of source code. The optimizer, when processing the profiling
information, will be able to use this extra column of information to
determine which basic block a given PC sample corresponds to.
Additional background material can be found on the DWARF wiki:
Proposed Changes to the DWARF Specification
Page and section numbers are keyed to the June 24, 2008 Working Draft
of the DWARF Version 4 specification.
In Section 6.2.2, "State Machine Registers," on Page 100, add the
following new register, after "column":
An unsigned integer identifying the basic block that
the current instruction belongs to. Discriminator
values are assigned arbitrarily by the DWARF producer
and serve to distinguish among several basic blocks
that may all be associated with the same source file,
line, and column. Where only one basic block exists for
a given source position, the discriminator value should
On page 101, add the following row to the table showing the initial
state of the registers at the beginning of each sequence, after
In Section 18.104.22.168, "Special Opcodes," on page 105, add the following step:
7. Set the discriminator register to 0.
Change "All of the special opcodes do those same six things" to "All
of the special opcodes do those same seven things".
In Section 22.214.171.124, "Standard Opcodes," on page 107, change the
description of DW_LNS_copy as follows:
The DW_LNS_copy opcode takes no operands... Then it sets
the discriminator register to 0, and sets the basic_block,
prologue_end and epilogue_begin registers to "false."
In Section 126.96.36.199, "Extended Opcodes," on page 111, add the following
new extended opcode:
The DW_LNE_set_discriminator opcode takes a single
parameter, an unsigned LEB128 integer. It sets the
discriminator register to the new value.
In Section 7.21, "Line Number Information," in Figure 38, on page 164,
add the following row:
Accepted Feb. 3, 2009 with following changes:
- Replace "basic block" with "block" (basic block has a more
- Add non-normative text describing use