[DebugInfo@O2][Dexter] Loop strength reduction preserves little debug info

[jmorse]

Bugzilla Link	38815
Version	trunk
OS	Linux
Blocks	#38116
CC	@chrisjbris,@gregbedwell,@CarlosAlbertoEnciso,@elavkje,@pogo59

Extended Description

It seems the loop-strength-reduction pass doesn't preserve debug information, for any transformation it makes. Compiling the example below with llvm/clang @ r340912 and options "-O2 -g -fno-inline" for x86_64, using both gdb and lldb, both the value of "i" and "blah" are reported as being optimised out, after being rewritten by LSR:

-------->8--------
int
foo(int blah)
{
int i;
for (i = blah; i < blah + 10; i++) {
if (i < 3)
return 0;
}
return 1;
}

int
main()
{
return foo(3);
}
--------8<--------

Which would be fairly frustrating if one landed in the loop and were unable to determine either the loop bound or which iteration it was on. (The example is contrived, but LSR still knackers more complicated loops).

This isn't a bug per se, and the DebugInfo doesn't mislead anyone, it's just a poor debug experience that will annoy developers. Particularly galling is that the values of "i" and "blah" remain in registers for the whole loop, but are still not available to the debugger.

[CarlosAlbertoEnciso]

For a better understanding:

-------->8--------
int foo(int blah) {
int bob;
for (bob = blah; bob < blah + 10; ++bob) {
if (bob < 3)
return 0;
}
return 1;
}
-------->8--------

*** IR Dump Before Loop Strength Reduction ***

; Preheader:
entry:
[..]
%cmp1 = icmp slt i32 %blah, 3, !dbg !​16
%0 = add nsw i32 %blah, 9, !dbg !​21
br label %for.body, !dbg !​22

; Loop:
for.body:
%bob.06 = phi i32 [ %blah, %entry ], [ %inc, %for.cond ]
call void @​llvm.dbg.value(metadata i32 %bob.06, metadata !​14, ...), !dbg !​15
call void @​llvm.dbg.value(metadata i32 %bob.06, metadata !​14, ...), !dbg !​15
br i1 %cmp1, label %cleanup, label %for.cond, !dbg !​26

for.cond:
%inc = add nsw i32 %bob.06, 1, !dbg !​23
call void @​llvm.dbg.value(metadata i32 %inc, metadata !​14, ...), !dbg !​15
call void @​llvm.dbg.value(metadata i32 %inc, metadata !​14, ...), !dbg !​15
%cmp = icmp slt i32 %bob.06, %0, !dbg !​23
br i1 %cmp, label %for.body, label %cleanup, !dbg !​22, !llvm.loop !​24

; Exit blocks
cleanup:
[..]

*** IR Dump After Loop Strength Reduction ***

; Preheader:
entry:
[..]
%cmp1 = icmp slt i32 %blah, 3, !dbg !​16
%0 = add nsw i32 %blah, 9, !dbg !​21
%1 = add i32 %blah, -1, !dbg !​22
br label %for.body, !dbg !​22

; Loop:
for.body:
%lsr.iv = phi i32 [ %lsr.iv.next, %for.cond ], [ %1, %entry ]
call void @​llvm.dbg.value(metadata i32 undef, metadata !​14, ...), !dbg !​15
call void @​llvm.dbg.value(metadata i32 undef, metadata !​14, ...), !dbg !​15
br i1 %cmp1, label %cleanup, label %for.cond, !dbg !​26

for.cond:
call void @​llvm.dbg.value(metadata i32 undef, metadata !​14, ...), !dbg !​15
call void @​llvm.dbg.value(metadata i32 undef, metadata !​14, ...), !dbg !​15
%lsr.iv.next = add i32 %lsr.iv, 1, !dbg !​23
%cmp = icmp slt i32 %lsr.iv.next, %0, !dbg !​23
br i1 %cmp, label %for.body, label %cleanup, !dbg !​22, !llvm.loop !​24

; Exit blocks
cleanup:
[..]

[CarlosAlbertoEnciso]

'bob' and 'blah' have been optimized out.

'bob' replaced by: '%lsr.iv' and '%lsr.iv.next'
'blah' replaced by: '%1'

[jmorse]

NB, Alexey made a start on this here: https://reviews.llvm.org/D67770

[markuslavin]

This issue is being addressed in https://reviews.llvm.org/D87494

[markuslavin]

This particular issue is resolved in commit 06758c6

[chrisjbris]

Please see https://reviews.llvm.org/D105207 for some more recent work in this area.

[chrisjbris]

Suggest this issue is closed as the above patch has been present since 13.0.0 and there is also D120168 to add further salvaging.

[llvmbot]

@llvm/issue-subscribers-debuginfo

[jryans]

As @chrisjbris suggested above, I'll go ahead and close this issue since LSR debug info salvaging has landed. We can track further refinements with new issues as needed.