[llvm-bugs] [Bug 44434] New: check_cfc: dash g generate different code with SelectionDAGISel

via llvm-bugs llvm-bugs at lists.llvm.org
Wed Jan 1 20:28:09 PST 2020


            Bug ID: 44434
           Summary: check_cfc: dash g generate different code with
           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: yechunliangcn at 163.com
                CC: llvm-bugs at lists.llvm.org

Based on the Bug 44422 - check_cfc: dash g generate different code with
machine-cp. There is another issue that the "isel" pass make different output
in the middle of the optimization, but the issue could not reproduce at the
end, would this be ignored? or is an potential issue.

More analysis about this issue:

$HOME/test-suite/MultiSource/Benchmarks/Bullet/btConvexConvexAlgorithm.cpp -w
-c -mllvm -opt-bisect-limit=669 -O2 -I
$HOME/test-suite/MultiSource/Benchmarks/Bullet/include -o out1.o 2>log1
clang++ -g
$HOME/test-suite/MultiSource/Benchmarks/Bullet/btConvexConvexAlgorithm.cpp -w
-c -mllvm -opt-bisect-limit=699 -O2 -I
$HOME/test-suite/MultiSource/Benchmarks/Bullet/include -o out2.o 2>log2
objdump -d -j .text out1.o > out1.obj
objdump -d -j .text out2.o > out2.obj
colordiff out1.obj out2.obj

[The case related with DAG Instruction Selection]
#Before X86 DAG->DAG Instruction Selection:
  %60 = load float, float* %12, align 4, !dbg !2196, !tbaa !2011
  call void @llvm.dbg.value(metadata float %60, metadata !2144, metadata
!DIExpression()), !dbg !2165
  %61 = fsub float %59, %60

# After X86 DAG->DAG Instruction Selection:
  %29:fr32 = MOVSSrm_alt %stack.0, 1, $noreg, 0, $noreg, debug-location
  DBG_VALUE %29:fr32, $noreg, !"capsuleRadiusB", !DIExpression()
  %28:fr32 = SUBSSrr killed %31:fr32(tied-def 0), killed %29:fr32, implicit

# Expectation:
  %28:fr32 = SUBSSrm killed %30:fr32(tied-def 0), %stack.0, 1, $noreg, 0,
$noreg, implicit $mxcsr :: (load 4 from %ir.12, !tbaa !46)


In the file lib/CodeGen/SelectionDAG/SelectionDAGISel.cpp,
and the function SelectAllBasicBlock(), 
while try to select the instruction with FastISel,
   FastIS>tryToFoldLoad() is not implemented because of the dbg_value which
block "fold the load", break by (!isFoldedOrDeadInstruction()).

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