probinson added a comment.
@rnk is correct that the phi's source location is propagating to the xorl. I
modified the .ll file to put a different line number on the phi, and the xorl
picked it up. This is clearly on the front-end to get right.
If we want to get down to cases, there are other poor choices of source
location in this tiny example. The statement source is
return a && b;
The 'a' is at column 10, the '&&' at 12, the 'b' at 15.
Now consider the load/test of 'a':
%0 = load i32, i32* %a.addr, align 4, !dbg !19
%tobool = icmp ne i32 %0, 0, !dbg !19
br i1 %tobool, label %land.rhs, label %land.end, !dbg !20
...
!19 = !DILocation(line: 3, column: 10, scope: !12)
!20 = !DILocation(line: 3, column: 12, scope: !12)
The load and icmp both use !19, the source location of 'a' in the expression;
the br uses !20, the '&&'. All seems good. Now consider the 'b' part:
%1 = load i32, i32* %b.addr, align 4, !dbg !21
%tobool1 = icmp ne i32 %1, 0, !dbg !20
br label %land.end
...
!20 = !DILocation(line: 3, column: 12, scope: !12)
!21 = !DILocation(line: 3, column: 15, scope: !12)
The load uses !21, 'b', but the icmp points to the '&&', and the br points
nowhere. This is at least inconsistent with how the LHS of the expression is
handled, and I would argue it's actually wrong.
This gets reflected in the generated code:
.loc 1 0 0 prologue_end # gdb-fail.c:0:0
xorl %eax, %eax ;;;; because of the phi's line-0
# kill: def $al killed $al killed $eax
.loc 1 3 10 # gdb-fail.c:3:10
cmpl $0, -4(%rbp) ;;;; load/compare of 'a' at
column 10
movb %al, -9(%rbp) # 1-byte Spill
.loc 1 3 12 is_stmt 0 # gdb-fail.c:3:12
je .LBB1_2 ;;;; the je attributed to the &&
# %bb.1: # %land.rhs
cmpl $0, -8(%rbp) ;;;; load/compare of 'b' at
column 12 (??) not 15
setne %al
movb %al, -9(%rbp) # 1-byte Spill
.LBB1_2: # %land.end
.loc 1 0 12 # gdb-fail.c:0:12
movb -9(%rbp), %al # 1-byte Reload
I'd also be happier if the reload was attributed to the same source location as
the following instructions, but that's probably a whole 'nother can of worms.
Repository:
rG LLVM Github Monorepo
CHANGES SINCE LAST ACTION
https://reviews.llvm.org/D91734/new/
https://reviews.llvm.org/D91734
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