[llvm-bugs] [Bug 34545] New: DBG_VALUE being lost in LiveDebugVariables

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=34545

            Bug ID: 34545
           Summary: DBG_VALUE being lost in LiveDebugVariables
           Product: libraries
           Version: trunk
          Hardware: PC
                OS: Linux
            Status: NEW
          Severity: enhancement
          Priority: P
         Component: Common Code Generator Code
          Assignee: unassignedbugs at nondot.org
          Reporter: karl-johan.karlsson at ericsson.com
                CC: llvm-bugs at lists.llvm.org

The program:

volatile int var = 8;
volatile unsigned sc = 1;
int main() {
  int bazinga = var;
  bazinga <<= sc;
  var = bazinga;
  return bazinga;
}

Compile with:
clang -S -emit-llvm -Xclang -O1 --target=x86_64-unknown-linux-gnu -g 'bar.c' -o
'./bar.ll'
opt -mcpu=x86-64 -O1 -S -o './bar.opt.ll' './bar.ll'
llc -O1 -filetype=obj -mtriple x86_64-unknown-linux-gnu -mcpu=x86-64 -o
'./bar.obj' './bar.opt.ll'


Adding the options -print-after-all and -debug to the llc binary show:

Before LiveDebugVariables:

# *** IR Dump After Machine Instruction Scheduler ***:
# Machine code for function main: NoPHIs, TracksLiveness

0B      BB#0: derived from LLVM BB %entry
16B             %vreg3<def> = MOV32rm %RIP, 1, %noreg, <ga:@var>, %noreg;
mem:Volatile LD4[@var](tbaa=!23)(dereferenceable) GR32:%vreg3 dbg:bar.c:4:17
                DBG_VALUE %vreg3, %noreg, !"bazinga", <!DIExpression()>;
GR32:%vreg3 line no:4
32B             %vreg1<def> = MOV32rm %RIP, 1, %noreg, <ga:@sc>, %noreg;
mem:Volatile LD4[@sc](tbaa=!23)(dereferenceable) GR32:%vreg1 dbg:bar.c:5:15
64B             %CL<def> = COPY %vreg1:sub_8bit; GR32:%vreg1 dbg:bar.c:5:11
96B             %vreg3<def,tied1> = SHL32rCL %vreg3<tied0>,
%EFLAGS<imp-def,dead>, %CL<imp-use,kill>; GR32:%vreg3 dbg:bar.c:5:11
                DBG_VALUE %vreg3, %noreg, !"bazinga", <!DIExpression()>;
GR32:%vreg3 line no:4
112B            MOV32mr %RIP, 1, %noreg, <ga:@var>, %noreg, %vreg3;
mem:Volatile ST4[@var](tbaa=!23) GR32:%vreg3 dbg:bar.c:6:7
128B            %EAX<def> = COPY %vreg3; GR32:%vreg3 dbg:bar.c:7:3
144B            RET 0, %EAX<kill>; dbg:bar.c:7:3

Debug variables are computed (the range for the variable bazinga ends before
slot idx 128):

********** COMPUTING LIVE DEBUG VARIABLES: main **********
********** DEBUG VARIABLES **********
!"bazinga,4"     [16r;128r):0 Loc0=%vreg3
# *** IR Dump After Debug Variable Analysis ***:
# Machine code for function main: NoPHIs, TracksLiveness

0B      BB#0: derived from LLVM BB %entry
16B             %vreg3<def> = MOV32rm %RIP, 1, %noreg, <ga:@var>, %noreg;
mem:Volatile LD4[@var](tbaa=!23)(dereferenceable) GR32:%vreg3 dbg:bar.c:4:17
32B             %vreg1<def> = MOV32rm %RIP, 1, %noreg, <ga:@sc>, %noreg;
mem:Volatile LD4[@sc](tbaa=!23)(dereferenceable) GR32:%vreg1 dbg:bar.c:5:15
64B             %CL<def> = COPY %vreg1:sub_8bit; GR32:%vreg1 dbg:bar.c:5:11
96B             %vreg3<def,tied1> = SHL32rCL %vreg3<tied0>,
%EFLAGS<imp-def,dead>, %CL<imp-use,kill>; GR32:%vreg3 dbg:bar.c:5:11
112B            MOV32mr %RIP, 1, %noreg, <ga:@var>, %noreg, %vreg3;
mem:Volatile ST4[@var](tbaa=!23) GR32:%vreg3 dbg:bar.c:6:7
128B            %EAX<def> = COPY %vreg3; GR32:%vreg3 dbg:bar.c:7:3
144B            RET 0, %EAX<kill>; dbg:bar.c:7:3

Later in the log we see this:

********** EMITTING LIVE DEBUG VARIABLES **********
!"bazinga,4"     [16r;128r):0 Loc0=%vreg3
        [16r;128r):0 BB#0-160B
# *** IR Dump After Virtual Register Rewriter ***:
# Machine code for function main: NoPHIs, TracksLiveness, NoVRegs

0B      BB#0: derived from LLVM BB %entry
16B             %EAX<def> = MOV32rm %RIP, 1, %noreg, <ga:@var>, %noreg;
mem:Volatile LD4[@var](tbaa=!23)(dereferenceable) dbg:bar.c:4:17
                DBG_VALUE %EAX, %noreg, !"bazinga", <!DIExpression()>; line
no:4
32B             %ECX<def> = MOV32rm %RIP, 1, %noreg, <ga:@sc>, %noreg;
mem:Volatile LD4[@sc](tbaa=!23)(dereferenceable) dbg:bar.c:5:15
64B             %CL<def> = KILL %CL, %ECX<imp-use,kill>; dbg:bar.c:5:11
96B             %EAX<def,tied1> = SHL32rCL %EAX<kill,tied0>,
%EFLAGS<imp-def,dead>, %CL<imp-use>; dbg:bar.c:5:11
112B            MOV32mr %RIP, 1, %noreg, <ga:@var>, %noreg, %EAX; mem:Volatile
ST4[@var](tbaa=!23) dbg:bar.c:6:7
144B            RET 0, %EAX; dbg:bar.c:7:3

The LiveDebugVariables pass has re-inserted DBG_VALUEs. But the DBG_VALUE after
the SHL32rCL is now gone.

This must be a fault, since DwarfDebug will end the range for a debug variable
when the physical reg is defined, or at basic block end. So I think that
LiveDebugVariables must insert new DBG_VALUE instructions whenever the physical
register that the debug variable is connected to is updated (within the slot
idx
range).

If trying the binary in gdb and trying to print the variable bazinga after the
shift it show <optimized out>.

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