[llvm-dev] A CFG issue

[Ning XIE via llvm-dev]

Hi,

I met an issue caused by Simplify the CFG. We have the following instructions:

sw.bb:                                            ; preds = %if.then63
  %bf.load65 = load i192, i192* %13, align 4
  %bf.lshr66 = lshr i192 %bf.load65, 80

sw.bb70:                                          ; preds = %if.then63
  %bf.load73 = load i192, i192* %15, align 4
 %bf.lshr74 = lshr i192 %bf.load73, 96

sw.bb78:                                          ; preds = %if.then63
 %bf.load81 = load i192, i192* %17, align 4
  %bf.lshr82 = lshr i192 %bf.load81, 112

sw.bb86:                                          ; preds = %if.then63
  %bf.load89 = load i192, i192* %19, align 4
  %bf.lshr90 = lshr i192 %bf.load89, 128

sw.bb94:                                          ; preds = %if.then63
  %bf.load97 = load i192, i192* %21, align 4
  %bf.lshr98 = lshr i192 %bf.load97, 144

Each pair of them is from different blocks and will be sunk into an end block. Also, a PHI node %.sink is created for the constant operand of lshr as

sw.epilog.sink.split:                             ; preds = %if.then63, %sw.bb, %sw.bb78, %sw.bb86, %sw.bb94
  %.sink = phi i192 [ 144, %sw.bb94 ], [ 128, %sw.bb86 ], [ 112, %sw.bb78 ], [ 80, %sw.bb ], [ 96, %if.then63 ]
  %bf.load97 = load i192, i192* %13, align 4
  %bf.lshr98 = lshr i192 %bf.load97, %.sink


Before lshr nodes are sunk, our lowering backend can understand which 32 bits of i192 are need and load that 32 bits only.
But after this CFG pass, %.sink is unknown, then there will be i192 load (8 32-bit loads).

I have some ideas how to handle it, but still I would like some feedback about what’s best way to do.
(The LLVM IR before and after CFG are shown below)

Thank you.

Best regards,
Ning Xie


*** Before simplify CFG is applied, we have the following LLVM IR ***

if.then63:                                        ; preds = %if.end
  %trunc = trunc i8 %11 to i3
  switch i3 %trunc, label %sw.epilog [
    i3 0, label %sw.bb
    i3 1, label %sw.bb70
    i3 2, label %sw.bb78
    i3 3, label %sw.bb86
    i3 -4, label %sw.bb94
  ]

sw.bb:                                            ; preds = %if.then63
  %13 = getelementptr inbounds %struct.C0000294C, %struct.C0000294C* %C0000159C, i32 0, i32 5, i32 3, i32 %conv50, i32 0
  %bf.load65 = load i192, i192* %13, align 4
  %bf.lshr66 = lshr i192 %bf.load65, 80
  %14 = trunc i192 %bf.lshr66 to i32
  %bf.cast68 = and i32 %14, 4095
  br label %sw.epilog

sw.bb70:                                          ; preds = %if.then63
  %15 = getelementptr inbounds %struct.C0000294C, %struct.C0000294C* %C0000159C, i32 0, i32 5, i32 3, i32 %conv50, i32 0
  %bf.load73 = load i192, i192* %15, align 4
  %bf.lshr74 = lshr i192 %bf.load73, 96
  %16 = trunc i192 %bf.lshr74 to i32
  %bf.cast76 = and i32 %16, 4095
  br label %sw.epilog

sw.bb78:                                          ; preds = %if.then63
  %17 = getelementptr inbounds %struct.C0000294C, %struct.C0000294C* %C0000159C, i32 0, i32 5, i32 3, i32 %conv50, i32 0
  %bf.load81 = load i192, i192* %17, align 4
  %bf.lshr82 = lshr i192 %bf.load81, 112
  %18 = trunc i192 %bf.lshr82 to i32
  %bf.cast84 = and i32 %18, 4095
  br label %sw.epilog

sw.bb86:                                          ; preds = %if.then63
  %19 = getelementptr inbounds %struct.C0000294C, %struct.C0000294C* %C0000159C, i32 0, i32 5, i32 3, i32 %conv50, i32 0
  %bf.load89 = load i192, i192* %19, align 4
  %bf.lshr90 = lshr i192 %bf.load89, 128
  %20 = trunc i192 %bf.lshr90 to i32
  %bf.cast92 = and i32 %20, 4095

sw.bb94:                                          ; preds = %if.then63
  %21 = getelementptr inbounds %struct.C0000294C, %struct.C0000294C* %C0000159C, i32 0, i32 5, i32 3, i32 %conv50, i32 0
  %bf.load97 = load i192, i192* %21, align 4
  %bf.lshr98 = lshr i192 %bf.load97, 144
  %22 = trunc i192 %bf.lshr98 to i32
  %bf.cast100 = and i32 %22, 4095
  br label %sw.epilog


*** IR Dump After Simplify the CFG ***

if.then63:                                        ; preds = %if.end
  %trunc = trunc i8 %11 to i3
  switch i3 %trunc, label %sw.epilog [
    i3 0, label %sw.bb
    i3 1, label %sw.epilog.sink.split
    i3 2, label %sw.bb78
    i3 3, label %sw.bb86
    i3 -4, label %sw.bb94
  ]

sw.bb:                                            ; preds = %if.then63
  br label %sw.epilog.sink.split

sw.bb78:                                          ; preds = %if.then63
  br label %sw.epilog.sink.split

sw.bb86:                                          ; preds = %if.then63
  br label %sw.epilog.sink.split

sw.bb94:                                          ; preds = %if.then63
  br label %sw.epilog.sink.split

sw.epilog.sink.split:                             ; preds = %if.then63, %sw.bb, %sw.bb78, %sw.bb86, %sw.bb94
  %.sink = phi i192 [ 144, %sw.bb94 ], [ 128, %sw.bb86 ], [ 112, %sw.bb78 ], [ 80, %sw.bb ], [ 96, %if.then63 ]
  %13 = getelementptr inbounds %struct.C0000294C, %struct.C0000294C* %C0000159C, i32 0, i32 5, i32 3, i32 %conv50, i32 0
  %bf.load97 = load i192, i192* %13, align 4
  %bf.lshr98 = lshr i192 %bf.load97, %.sink
  %14 = trunc i192 %bf.lshr98 to i32
  %bf.cast100 = and i32 %14, 4095
  br label %sw.epilog

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