[LLVMbugs] [Bug 13799] New: Poor optimization of bitfield initialization

[bugzilla-daemon at llvm.org]

http://llvm.org/bugs/show_bug.cgi?id=13799

             Bug #: 13799
           Summary: Poor optimization of bitfield initialization
           Product: new-bugs
           Version: trunk
          Platform: PC
        OS/Version: Linux
            Status: NEW
          Severity: enhancement
          Priority: P
         Component: new bugs
        AssignedTo: unassignedbugs at nondot.org
        ReportedBy: baldrick at free.fr
                CC: llvmbugs at cs.uiuc.edu
    Classification: Unclassified


Created attachment 9184
  --> http://llvm.org/bugs/attachment.cgi?id=9184
unoptimized testcase .ll

This testcase could be optimized to a succession of three i8 stores of -1 to
successive memory locations.  In particular all the loads just load bits which
were either stored to just a few lines before, or are uninitialized, so could
be computed at compile time - but they aren't.  In short it should be possible
to optimize down to

define void @bar() nounwind uwtable {
entry:
  %foo = alloca %struct.Foo, align 4
  %0 = getelementptr %struct.Foo* %foo, i64 0, i32 1
  store i8 -1, i8* %0, align 4
  %1 = getelementptr %struct.Foo* %foo, i64 0, i32 2
  store i8 -1, i8* %1, align 1
  %2 = getelementptr %struct.Foo* %foo, i64 0, i32 3
  store i8 -1, i8* %2, align 2
  call void @baz(%struct.Foo* %foo) nounwind
  ret void
}

Currently -std-compile-opts gives:


define void @bar(...) nounwind uwtable {
entry:
  %foo = alloca %struct.Foo, align 8
  %0 = getelementptr inbounds %struct.Foo* %foo, i64 0, i32 1
  store i8 -1, i8* %0, align 4
  %1 = bitcast i8* %0 to i16*
  %2 = load i16* %1, align 4
  %3 = or i16 %2, 448
  store i16 %3, i16* %1, align 4
  %4 = getelementptr inbounds %struct.Foo* %foo, i64 0, i32 2
  %5 = lshr i16 %3, 8
  %6 = trunc i16 %5 to i8
  %7 = or i8 %6, 126
  store i8 %7, i8* %4, align 1
  %8 = bitcast i8* %4 to i16*
  %9 = load i16* %8, align 1
  %10 = or i16 %9, 896
  store i16 %10, i16* %8, align 1
  %11 = getelementptr inbounds %struct.Foo* %foo, i64 0, i32 3
  %12 = lshr i16 %10, 8
  %13 = trunc i16 %12 to i8
  %14 = or i8 %13, 28
  store i8 %14, i8* %11, align 2
  call void @baz(%struct.Foo* %foo) nounwind
  ret void
}

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