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Extending variable live ranges in more cases #77439

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@BrianBohe BrianBohe commented Oct 25, 2022

While debugging Issue 47202, I found that variables can die and born again without an instruction being emitted nor the emitter moving.

Here is an example from #77289 (comment):

I found a case where a variable (V06) dies (in [000054]) at the beginning of a basic block (BB05), and becomes alive (in [000016]) before an instruction is emitter. We would want to extend the live ranges in this case:

                                                                        /--*  t52    long   
Generating: N087 ( 10, 16) [000054] DA---------                         *  STORE_LCL_VAR long   V06 loc5         d:3 rcx REG rcx
							V06 in reg rcx is becoming live  [000054]
							Live regs: 00000000 {} => 00000002 {rcx}
							Live vars: {} => {V06}
New debug range: first
IN000f:        jmp      L_M3923_BB06

Variable Live Range History Dump for BB04
V04 loc3: rdx [(G_M3923_IG03,ins#5,ofs#27), (G_M3923_IG04,ins#1,ofs#10)]
V06 loc5: rcx [(G_M3923_IG04,ins#2,ofs#13), ...]

=============== Generating BB05 [060..06B), preds={BB01} succs={BB06} flags=0x00000000.20010020: i label LIR 
BB05 IN (1)={V09    }
     OUT(1)={    V06}

Recording Var Locations at start of BB05
  V09(rcx)
Change life 0000000000000008 {V06} -> 0000000000000002 {V09}
							V06 in reg rcx is becoming dead  [------]
							Live regs: (unchanged) 00000000 {}
							V09 in reg rcx is becoming live  [------]
							Live regs: 00000000 {} => 00000002 {rcx}
							Live regs: (unchanged) 00000002 {rcx}
							GC regs: (unchanged) 00000000 {}
							Byref regs: (unchanged) 00000000 {}

      L_M3923_BB05:

      G_M3923_IG04:        ; offs=000044H, funclet=00, bbWeight=0.50, byref
Mapped BB05 to G_M3923_IG05
Label: IG05, GCvars=0000000000000000 {}, gcrefRegs=00000000 {}, byrefRegs=00000000 {}

Scope info: begin block BB05, IL range [060..06B)
Added IP mapping: 0x0061 STACK_EMPTY (G_M3923_IG05,ins#0,ofs#0) label
Generating: N091 (???,???) [000093] -----------                            IL_OFFSET void   INLRT @ 0x061[E-] REG NA
Generating: N093 (  1,  1) [000086] -----------                   t86 =    LCL_VAR   long   V09 cse0         u:1 rcx (last use) REG rcx <l:$100, c:$140>
                                                                        /--*  t86    long   
Generating: N095 (  5,  4) [000016] DA--G------                         *  STORE_LCL_VAR long   V06 loc5         d:2 rcx REG rcx
							V09 in reg rcx is becoming dead  [000086]
							Live regs: 00000002 {rcx} => 00000000 {}
							Live vars: {V09} => {}
							V06 in reg rcx is becoming live  [000016]
							Live regs: 00000000 {} => 00000002 {rcx}
							Live vars: {} => {V06}
New debug range: new var or location

Extending variable live ranges in more cases
9755e96 
When the emitter moved to the next group but has not
emitted any instruction, and the variable died and
becomes alive again, we would like to extend its range.
@ghost ghost added the area-CodeGen-coreclr CLR JIT compiler in src/coreclr/src/jit and related components such as SuperPMI label Oct 25, 2022
@ghost ghost assigned BrianBohe Oct 25, 2022
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ghost commented Oct 25, 2022

Tagging subscribers to this area: @JulieLeeMSFT, @jakobbotsch
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Issue Details

While debugging Issue 47202, I found that variables can die and born again without an instruction being emitted nor the emitter moving.

Here is an example from #77289 (comment):

I found a case where a variable (V06) dies (in [000054]) at the beginning of a basic block (BB05), and becomes alive (in [000016]) before an instruction is emitter. We would want to extend the live ranges in this case:

                                                                        /--*  t52    long   
Generating: N087 ( 10, 16) [000054] DA---------                         *  STORE_LCL_VAR long   V06 loc5         d:3 rcx REG rcx
							V06 in reg rcx is becoming live  [000054]
							Live regs: 00000000 {} => 00000002 {rcx}
							Live vars: {} => {V06}
New debug range: first
IN000f:        jmp      L_M3923_BB06

Variable Live Range History Dump for BB04
V04 loc3: rdx [(G_M3923_IG03,ins#5,ofs#27), (G_M3923_IG04,ins#1,ofs#10)]
V06 loc5: rcx [(G_M3923_IG04,ins#2,ofs#13), ...]

=============== Generating BB05 [060..06B), preds={BB01} succs={BB06} flags=0x00000000.20010020: i label LIR 
BB05 IN (1)={V09    }
     OUT(1)={    V06}

Recording Var Locations at start of BB05
  V09(rcx)
Change life 0000000000000008 {V06} -> 0000000000000002 {V09}
							V06 in reg rcx is becoming dead  [------]
							Live regs: (unchanged) 00000000 {}
							V09 in reg rcx is becoming live  [------]
							Live regs: 00000000 {} => 00000002 {rcx}
							Live regs: (unchanged) 00000002 {rcx}
							GC regs: (unchanged) 00000000 {}
							Byref regs: (unchanged) 00000000 {}

      L_M3923_BB05:

      G_M3923_IG04:        ; offs=000044H, funclet=00, bbWeight=0.50, byref
Mapped BB05 to G_M3923_IG05
Label: IG05, GCvars=0000000000000000 {}, gcrefRegs=00000000 {}, byrefRegs=00000000 {}

Scope info: begin block BB05, IL range [060..06B)
Added IP mapping: 0x0061 STACK_EMPTY (G_M3923_IG05,ins#0,ofs#0) label
Generating: N091 (???,???) [000093] -----------                            IL_OFFSET void   INLRT @ 0x061[E-] REG NA
Generating: N093 (  1,  1) [000086] -----------                   t86 =    LCL_VAR   long   V09 cse0         u:1 rcx (last use) REG rcx <l:$100, c:$140>
                                                                        /--*  t86    long   
Generating: N095 (  5,  4) [000016] DA--G------                         *  STORE_LCL_VAR long   V06 loc5         d:2 rcx REG rcx
							V09 in reg rcx is becoming dead  [000086]
							Live regs: 00000002 {rcx} => 00000000 {}
							Live vars: {V09} => {}
							V06 in reg rcx is becoming live  [000016]
							Live regs: 00000000 {} => 00000002 {rcx}
							Live vars: {} => {V06}
New debug range: new var or location
Author: BrianBohe
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area-CodeGen-coreclr
Milestone: -

Updating case in same instruction group
e7c2cb5 
Just to be consistent with new method objective and description.
@BrianBohe BrianBohe closed this Oct 25, 2022
@BrianBohe BrianBohe reopened this Oct 25, 2022
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BrianBohe commented Oct 25, 2022

After this PR

Variable Live Range History Dump for BB04
V04 loc3: rdx [(G_M3923_IG03,ins#8,ofs#46), (G_M3923_IG04,ins#1,ofs#10)]
V06 loc5: rcx [(G_M3923_IG04,ins#2,ofs#13), ...]

=============== Generating BB05 [060..06B), preds={BB01} succs={BB06} flags=0x00000000.20010020: i label LIR 
BB05 IN (1)={V09    }
     OUT(1)={    V06}

Recording Var Locations at start of BB05
  V09(rcx)
Change life 0000000000000008 {V06} -> 0000000000000002 {V09}
							V06 in reg rcx is becoming dead  [------]
							Live regs: (unchanged) 00000000 {}
							V09 in reg rcx is becoming live  [------]
							Live regs: 00000000 {} => 00000002 {rcx}
							Live regs: (unchanged) 00000002 {rcx}
							GC regs: (unchanged) 00000000 {}
							Byref regs: (unchanged) 00000000 {}

      L_M3923_BB05:

      G_M3923_IG04:        ; offs=000044H, funclet=00, bbWeight=0.50, byref
Mapped BB05 to G_M3923_IG05
Label: IG05, GCvars=0000000000000000 {}, gcrefRegs=00000000 {}, byrefRegs=00000000 {}

Scope info: begin block BB05, IL range [060..06B)
Added IP mapping: 0x0061 STACK_EMPTY (G_M3923_IG05,ins#0,ofs#0) label
Generating: N091 (???,???) [000093] -----------                            IL_OFFSET void   INLRT @ 0x061[E-] REG NA
Generating: N093 (  1,  1) [000086] -----------                   t86 =    LCL_VAR   long   V09 cse0         u:1 rcx (last use) REG rcx <l:$100, c:$140>
                                                                        /--*  t86    long   
Generating: N095 (  5,  4) [000016] DA--G------                         *  STORE_LCL_VAR long   V06 loc5         d:2 rcx REG rcx
							V09 in reg rcx is becoming dead  [000086]
							Live regs: 00000002 {rcx} => 00000000 {}
							Live vars: {V09} => {}
							V06 in reg rcx is becoming live  [000016]
							Live regs: 00000000 {} => 00000002 {rcx}
							Live vars: {} => {V06}
Extending debug range...

Variable Live Range History Dump for BB05
V06 loc5: rcx [(G_M3923_IG04,ins#2,ofs#13), ...]

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AndyAyersMS commented Oct 26, 2022

I wonder if we should consider moving to a "lazy kill" mode like we do for gc tracking. That is, even if a local is no longer live, so long as the register it was in does not get modified, we can still report the local in the debug info.

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BrianBohe commented Oct 26, 2022

I wonder if we should consider moving to a "lazy kill" mode like we do for gc tracking. That is, even if a local is no longer live, so long as the register it was in does not get modified, we can still report the local in the debug info.

I didn't know that such a thing exists, sounds like the appropriate case. Let me read about it a little bit.

@BrianBohe BrianBohe marked this pull request as draft November 16, 2022 23:07
@ghost ghost closed this Dec 17, 2022
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ghost commented Dec 17, 2022

Draft Pull Request was automatically closed for 30 days of inactivity. Please let us know if you'd like to reopen it.

@ghost ghost locked as resolved and limited conversation to collaborators Jan 16, 2023
@BrianBohe BrianBohe deleted the updating_variable_live_ranges_extension branch February 10, 2023 23:06
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