[llvm-bugs] [Bug 39154] New: Bad machine code: Instruction loads from dead spill slot

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=39154

            Bug ID: 39154
           Summary: Bad machine code: Instruction loads from dead spill
                    slot
           Product: new-bugs
           Version: trunk
          Hardware: PC
                OS: Linux
            Status: NEW
          Severity: enhancement
          Priority: P
         Component: new bugs
          Assignee: unassignedbugs at nondot.org
          Reporter: bjorn.a.pettersson at ericsson.com
                CC: llvm-bugs at lists.llvm.org

Created attachment 20951
  --> https://bugs.llvm.org/attachment.cgi?id=20951&action=edit
reduced reproducer

Running the attached reproducer gives:

llc  -run-pass greedy -verify-regalloc -mtriple x86_64-- -o -
remat-undef-use.mir 

********** MACHINEINSTRS **********
# Machine code for function test: IsSSA, NoPHIs, TracksLiveness
Frame Objects:
  fi#-1: size=1504, align=8, fixed, at location [SP+8]
  fi#0: size=4, align=4, at location [SP+8]

0B      bb.0:
          successors: %bb.1(0x80000000); %bb.1(100.00%)

16B       dead %3:gr32 = MOV32rm $rsp, 1, $noreg, 8, $noreg :: (load 4 from
%fixed-stack.0, align 16)
32B       %1:gr32 = MOV32rm %stack.0, 1, $noreg, 0, $noreg :: (load 4 from
%stack.0)

48B     bb.1:
        ; predecessors: %bb.0, %bb.1
          successors: %bb.1(0x80000000); %bb.1(100.00%)

56B       %2:gr32 = MOV32rm $rsp, 1, $noreg, 8, $noreg :: (load 4 from
%fixed-stack.0, align 16)
64B       INLINEASM &nop [sideeffect] [attdialect],
$0:[regdef:GR32_ABCD_and_GR32_BSI], implicit %2:gr32
80B       INLINEASM &nop [sideeffect] [attdialect], $0:[clobber], implicit-def
dead early-clobber $rax, $1:[clobber], implicit-def dead early-clobber $rbx,
$2:[clobber], implicit-def dead early-clobber $rcx, $3:[clobber], implicit-def
dead early-clobber $rdx, $4:[clobber], implicit-def dead early-clobber $rsi,
$5:[clobber], implicit-def dead early-clobber $rdi, $6:[clobber], implicit-def
dead early-clobber $rbp, $7:[clobber], implicit-def dead early-clobber $r8,
$8:[clobber], implicit-def dead early-clobber $r9, $9:[clobber], implicit-def
dead early-clobber $r10, $10:[clobber], implicit-def dead early-clobber $r11,
$11:[clobber], implicit-def dead early-clobber $r12, $12:[clobber],
implicit-def dead early-clobber $r13, $13:[clobber], implicit-def dead
early-clobber $r14, $14:[clobber], implicit-def dead early-clobber $r15
96B       JMP_1 %bb.1

# End machine code for function test.

*** Bad machine code: Instruction loads from dead spill slot ***
- function:    test
- basic block: %bb.0  (0x4514ef8) [0B;48B)
- instruction: 32B      %1:gr32 = MOV32rm %stack.0, 1, $noreg, 0, $noreg ::
(load 4 from %stack.0)
- operand 1:   %stack.0
Live stack: SS#0 EMPTY  0 at x weight:0.000000e+00

*** Bad machine code: Instruction ending live segment on dead slot has no dead
flag ***
- function:    test
- basic block: %bb.0  (0x4514ef8) [0B;48B)
- instruction: 32B      %1:gr32 = MOV32rm %stack.0, 1, $noreg, 0, $noreg ::
(load 4 from %stack.0)
- liverange:   [32r,32d:0)  0 at 32r
- v. register: %1
- segment:     [32r,32d:0)
LLVM ERROR: Found 2 machine code errors.


The problem with "Instruction ending live segment on dead slot has no dead
flag" might be a duplicate of https://bugs.llvm.org/show_bug.cgi?id=33024


Running with -debug we can see this:

Inline spilling GR32:%0 [16r,112B:0)  0 at 16r weight:4.188229e+00
>From original %0
        remat:  56r     %2:gr32 = MOV32rm $rsp, 1, $noreg, 8, $noreg :: (load 4
from %fixed-stack.0, align 16)
                64e     INLINEASM &nop [sideeffect] [attdialect],
$0:[regdef:GR32_ABCD_and_GR32_BSI], implicit killed %2:gr32

All defs dead: dead %0:gr32 = MOV32rm $rsp, 1, $noreg, 8, $noreg :: (load 4
from %fixed-stack.0, align 16)
Remat created 1 dead defs.
Deleting dead def 16r   dead %0:gr32 = MOV32rm $rsp, 1, $noreg, 8, $noreg ::
(load 4 from %fixed-stack.0, align 16)
1 registers to spill after remat.
Merged spilled regs: SS#0 EMPTY  0 at x weight:0.000000e+00
spillAroundUses %0
        folded:   32r   %1:gr32 = MOV32rm %stack.0, 1, $noreg, 0, $noreg ::
(load 4 from %stack.0)



Afaict InlineSpiller is trying to add remats for the fixed-stack load. So it
inserts a load before the use inside the loop (start of bb.1), but since the
use in the COPY (in bb.0) is undef it isn't replaced by a remat, instead we get
a reload from a new spill slot. Although, there is no spill to the spill slot
(perhaps because the code thinks that we rematerialized all uses(?)).

Not sure where to solve this. Isn't it unfortunate to have these undef uses
(and dead defs) lingering around when doing the register allocation? Maybe we
could cleanup up those at the very beginning? Or should the remat be smarter
and replace the undef COPY with an IMPLICIT_DEF?

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