1355136 - LUL on i686/x86_64-linux: allow CFAs to be arbitrary prefix expressions

Status: RESOLVED FIXED

(Core :: Gecko Profiler, defect)

Description

[Julian Seward [:jseward]]

For reasons which are unclear, but possibly due to lack of any known use cases
when the code was written, LUL on i686/x86_64-linux only accepts CFA (canonical
frame address) expressions of the form SP+offset or FP+offset.  However, on
Fedora 25 x86_64 and Ubuntu 16.10 x86_64, at least one address range per object
uses a Dwarf expression for the CFA, for example:

00000018 0000000000000024 0000001c FDE cie=00000000 pc=00000000000031e0..00000000000031f0
  DW_CFA_def_cfa_offset: 16
  DW_CFA_advance_loc: 6 to 00000000000031e6
  DW_CFA_def_cfa_offset: 24
  DW_CFA_advance_loc: 10 to 00000000000031f0
  DW_CFA_def_cfa_expression (DW_OP_breg7 (rsp): 8; DW_OP_breg16 (rip): 0; DW_OP_lit15; DW_OP_and; DW_OP_lit11; DW_OP_ge; DW_OP_lit3; DW_OP_shl; DW_OP_plus)
  DW_CFA_nop
  DW_CFA_nop
  DW_CFA_nop
  DW_CFA_nop

producing the following complaint from LUL:

[Main Thread]: V/prof [32573] LUL  can't summarise: SVMA=0x31f0: rule for DW_REG_CFA: invalid |how|, expr=LExpr(PFXEXPR,0,0)

Given that LUL is perfectly capable of handling such a CFA expression, it seems
artificial to stop it doing so.

Comment 1

[Julian Seward [:jseward]]

Attachment: LUL on i686/x86_64-linux: allow CFAs to be arbitrary prefix expressions

For reasons which are unclear, but possibly due to lack of any known use cases
when the code was written, LUL on i686/x86_64-linux only accepts CFA (canonical
frame address) expressions of the form SP+offset or FP+offset.  However, on
Fedora 25 x86_64 and Ubuntu 16.10 x86_64, at least one address range per object
uses a Dwarf expression for the CFA, for example:

00000018 000000000024 0000001c FDE cie=00000000 pc=0000000031e0..0000000031f0
  DW_CFA_def_cfa_offset: 16
  DW_CFA_advance_loc: 6 to 00000000000031e6
  DW_CFA_def_cfa_offset: 24
  DW_CFA_advance_loc: 10 to 00000000000031f0
  DW_CFA_def_cfa_expression(
          DW_OP_breg7 (rsp): 8; DW_OP_breg16 (rip): 0; DW_OP_lit15; DW_OP_and;
          DW_OP_lit11; DW_OP_ge; DW_OP_lit3; DW_OP_shl; DW_OP_plus)

producing the following complaint from LUL:

  can't summarise: SVMA=0x31f0: rule for DW_REG_CFA: invalid |how|, expr=LExpr(PFXEXPR,0,0)

Given that LUL is capable of handling such a CFA expression, it seems artificial
to stop it doing so.  This patch changes Summariser::Rule() so as to allow such
expressions.

Comment 2

[Nathan Froyd [:froydnj]]

Comment on attachment 8857346 [details] [diff] [review]
LUL on i686/x86_64-linux: allow CFAs to be arbitrary prefix expressions

Review of attachment 8857346 [details] [diff] [review]:
-----------------------------------------------------------------

You know, I was just wondering yesterday how they expressed dynamic stack alignment in the dwarf unwind rules, and there you go.

Comment 3

[Pulsebot]

Pushed by [e-mail]:
https://hg.mozilla.org/integration/mozilla-inbound/rev/93166de3c8f8
LUL on i686/x86_64-linux: allow CFAs to be arbitrary prefix expressions.  r=froydnj.

Comment 4

[Sebastian Hengst [:aryx] (needinfo me if it's about an intermittent or backout)]

https://hg.mozilla.org/mozilla-central/rev/93166de3c8f8