[llvm-bugs] [Bug 31697] New: Bitcasts block copy forwarding

[via llvm-bugs]

https://llvm.org/bugs/show_bug.cgi?id=31697

            Bug ID: 31697
           Summary: Bitcasts block copy forwarding
           Product: libraries
           Version: trunk
          Hardware: All
                OS: All
            Status: NEW
          Severity: normal
          Priority: P
         Component: Global Analyses
          Assignee: unassignedbugs at nondot.org
          Reporter: arcata at gmail.com
                CC: llvm-bugs at lists.llvm.org
    Classification: Unclassified

If you have a situation where memory pointed to by a readonly argument %x is
copied to a local alloca %y and then %y is projected from, then LLVM normally
eliminates the unnecessary copy. This IR:

declare void @llvm.memcpy.p0i8.p0i8.i32(i8*, i8*, i32, i32, i1)
declare void @llvm.lifetime.start(i64, i8*)
declare void @llvm.lifetime.end(i64, i8*)

define i32 @copy_then_extract({ i32,i32,i32,i32 }* noalias nocapture readonly
dereferenceable(16) %x, i32 %n) {
entry:
  %y = alloca { i32, i32, i32, i32 }, align 16
  %x0p = getelementptr inbounds { i32, i32, i32, i32 }, { i32, i32, i32, i32 }*
%x, i32 0, i32 0
  %x0 = load i32, i32* %x0p
  %x1p = getelementptr inbounds { i32, i32, i32, i32 }, { i32, i32, i32, i32 }*
%x, i32 0, i32 1
  %x1 = load i32, i32* %x1p
  %x2p = getelementptr inbounds { i32, i32, i32, i32 }, { i32, i32, i32, i32 }*
%x, i32 0, i32 2
  %x2 = load i32, i32* %x2p
  %x3p = getelementptr inbounds { i32, i32, i32, i32 }, { i32, i32, i32, i32 }*
%x, i32 0, i32 3
  %x3 = load i32, i32* %x3p

  %yi8 = bitcast { i32, i32, i32, i32 }* %y to i8*
  call void @llvm.lifetime.start(i64 16, i8* %yi8)

  %y0p = getelementptr inbounds { i32, i32, i32, i32 }, { i32, i32, i32, i32 }*
%y, i32 0, i32 0
  store i32 %x0, i32* %y0p
  %y1p = getelementptr inbounds { i32, i32, i32, i32 }, { i32, i32, i32, i32 }*
%y, i32 0, i32 1
  store i32 %x1, i32* %y1p
  %y2p = getelementptr inbounds { i32, i32, i32, i32 }, { i32, i32, i32, i32 }*
%y, i32 0, i32 2
  store i32 %x2, i32* %y2p
  %y3p = getelementptr inbounds { i32, i32, i32, i32 }, { i32, i32, i32, i32 }*
%y, i32 0, i32 3
  store i32 %x3, i32* %y3p

  %ya = bitcast i8* %yi8 to i32*
  %ya2p = getelementptr inbounds i32, i32* %ya, i64 2
  %y2 = load i32, i32* %ya2p

  %yi8_2 = bitcast { i32, i32, i32, i32 }* %y to i8*
  call void @llvm.lifetime.end(i64 16, i8* %yi8_2)
  ret i32 %y2
}

reduces to:

define i32 @copy_then_extract({ i32, i32, i32, i32 }* noalias nocapture
readonly dereferenceable(16) %x, i32 %n) local_unnamed_addr #0 {
entry:
  %x2p = getelementptr inbounds { i32, i32, i32, i32 }, { i32, i32, i32, i32 }*
%x, i64 0, i32 2
  %x2 = load i32, i32* %x2p, align 4
  ret i32 %x2
}

However, if %ya2p becomes a dynamic GEP by changing `i64 2` to `i64 %n`, then
the bitcast can no longer be reduced since that's not valid against a struct.
This ends up completely blocking copy forwarding. This IR:

declare void @llvm.memcpy.p0i8.p0i8.i32(i8*, i8*, i32, i32, i1)
declare void @llvm.lifetime.start(i64, i8*)
declare void @llvm.lifetime.end(i64, i8*)

define i32 @copy_then_extract({ i32,i32,i32,i32 }* noalias nocapture readonly
dereferenceable(16) %x, i32 %n) {
entry:
  %y = alloca { i32, i32, i32, i32 }, align 16
  %x0p = getelementptr inbounds { i32, i32, i32, i32 }, { i32, i32, i32, i32 }*
%x, i32 0, i32 0
  %x0 = load i32, i32* %x0p
  %x1p = getelementptr inbounds { i32, i32, i32, i32 }, { i32, i32, i32, i32 }*
%x, i32 0, i32 1
  %x1 = load i32, i32* %x1p
  %x2p = getelementptr inbounds { i32, i32, i32, i32 }, { i32, i32, i32, i32 }*
%x, i32 0, i32 2
  %x2 = load i32, i32* %x2p
  %x3p = getelementptr inbounds { i32, i32, i32, i32 }, { i32, i32, i32, i32 }*
%x, i32 0, i32 3
  %x3 = load i32, i32* %x3p

  %yi8 = bitcast { i32, i32, i32, i32 }* %y to i8*
  call void @llvm.lifetime.start(i64 16, i8* %yi8)

  %y0p = getelementptr inbounds { i32, i32, i32, i32 }, { i32, i32, i32, i32 }*
%y, i32 0, i32 0
  store i32 %x0, i32* %y0p
  %y1p = getelementptr inbounds { i32, i32, i32, i32 }, { i32, i32, i32, i32 }*
%y, i32 0, i32 1
  store i32 %x1, i32* %y1p
  %y2p = getelementptr inbounds { i32, i32, i32, i32 }, { i32, i32, i32, i32 }*
%y, i32 0, i32 2
  store i32 %x2, i32* %y2p
  %y3p = getelementptr inbounds { i32, i32, i32, i32 }, { i32, i32, i32, i32 }*
%y, i32 0, i32 3
  store i32 %x3, i32* %y3p

  %ya = bitcast i8* %yi8 to i32*
  %ya2p = getelementptr inbounds i32, i32* %ya, i32 %n
  %y2 = load i32, i32* %ya2p

  %yi8_2 = bitcast { i32, i32, i32, i32 }* %y to i8*
  call void @llvm.lifetime.end(i64 16, i8* %yi8_2)
  ret i32 %y2
}

still keeps the unnecessary copy in the optimized IR:

define i32 @copy_then_extract({ i32, i32, i32, i32 }* noalias nocapture
readonly dereferenceable(16) %x, i32 %n) local_unnamed_addr #1 {
entry:
  %y = alloca <4 x i32>, align 16
  %0 = bitcast { i32, i32, i32, i32 }* %x to <4 x i32>*
  %1 = load <4 x i32>, <4 x i32>* %0, align 4
  %yi8 = bitcast <4 x i32>* %y to i8*
  call void @llvm.lifetime.start(i64 16, i8* %yi8)
  store <4 x i32> %1, <4 x i32>* %y, align 16
  %2 = sext i32 %n to i64
  %ya2p = getelementptr inbounds <4 x i32>, <4 x i32>* %y, i64 0, i64 %2
  %y2 = load i32, i32* %ya2p, align 4
  call void @llvm.lifetime.end(i64 16, i8* %yi8)
  ret i32 %y2
}

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