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Optimize switch lookup tables with linear mapping.

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This is a simple optimization for switch table lookup:
It computes the output value directly with an (optional) mul and add if there is a linear mapping between index and output.
Example:

int f1(int x) {
  switch (x) {
    case 0: return 10;
    case 1: return 11;
    case 2: return 12;
    case 3: return 13;
  }
  return 0;
}

generates:

define i32 @f1(i32 %x) #0 {
entry:
  %0 = icmp ult i32 %x, 4
  br i1 %0, label %switch.lookup, label %return

switch.lookup:
  %switch.offset = add i32 %x, 10
  ret i32 %switch.offset

return:
  ret i32 0
}



git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@222121 91177308-0d34-0410-b5e6-96231b3b80d8
This commit is contained in:
Erik Eckstein
2014-11-17 09:13:57 +00:00
parent 17e95ead36
commit 72a1394991
2 changed files with 160 additions and 2 deletions
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103
test/Transforms/SimplifyCFG/X86/switch_to_lookup_table.ll
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@@ -895,7 +895,7 @@ sw.bb1: br label %return
sw.bb2: br label %return
sw.default: br label %return
return:
%x = phi i32 [ 3, %sw.default ], [ 5, %sw.bb2 ], [ 7, %sw.bb1 ], [ 9, %entry ]
%x = phi i32 [ 3, %sw.default ], [ 5, %sw.bb2 ], [ 7, %sw.bb1 ], [ 10, %entry ]
ret i32 %x
; CHECK-LABEL: @threecases(
; CHECK-NOT: switch i32
@@ -977,3 +977,104 @@ return:
; CHECK: switch i32
; CHECK-NOT: @switch.table
}
; We can use linear mapping.
define i8 @linearmap1(i32 %c) {
entry:
switch i32 %c, label %sw.default [
i32 10, label %return
i32 11, label %sw.bb1
i32 12, label %sw.bb2
i32 13, label %sw.bb3
]
sw.bb1: br label %return
sw.bb2: br label %return
sw.bb3: br label %return
sw.default: br label %return
return:
%x = phi i8 [ 3, %sw.default ], [ 3, %sw.bb3 ], [ 8, %sw.bb2 ], [ 13, %sw.bb1 ], [ 18, %entry ]
ret i8 %x
; CHECK-LABEL: @linearmap1(
; CHECK: entry:
; CHECK-NEXT: %switch.tableidx = sub i32 %c, 10
; CHECK: switch.lookup:
; CHECK-NEXT: %switch.idx.cast = trunc i32 %switch.tableidx to i8
; CHECK-NEXT: %switch.idx.mult = mul i8 %switch.idx.cast, -5
; CHECK-NEXT: %switch.offset = add i8 %switch.idx.mult, 18
; CHECK-NEXT: ret i8 %switch.offset
}
; Linear mapping in a different configuration.
define i32 @linearmap2(i8 %c) {
entry:
switch i8 %c, label %sw.default [
i8 -10, label %return
i8 -11, label %sw.bb1
i8 -12, label %sw.bb2
i8 -13, label %sw.bb3
]
sw.bb1: br label %return
sw.bb2: br label %return
sw.bb3: br label %return
sw.default: br label %return
return:
%x = phi i32 [ 3, %sw.default ], [ 18, %sw.bb3 ], [ 19, %sw.bb2 ], [ 20, %sw.bb1 ], [ 21, %entry ]
ret i32 %x
; CHECK-LABEL: @linearmap2(
; CHECK: entry:
; CHECK-NEXT: %switch.tableidx = sub i8 %c, -13
; CHECK: switch.lookup:
; CHECK-NEXT: %switch.idx.cast = zext i8 %switch.tableidx to i32
; CHECK-NEXT: %switch.offset = add i32 %switch.idx.cast, 18
; CHECK-NEXT: ret i32 %switch.offset
}
; Linear mapping with overflows.
define i8 @linearmap3(i32 %c) {
entry:
switch i32 %c, label %sw.default [
i32 10, label %return
i32 11, label %sw.bb1
i32 12, label %sw.bb2
i32 13, label %sw.bb3
]
sw.bb1: br label %return
sw.bb2: br label %return
sw.bb3: br label %return
sw.default: br label %return
return:
%x = phi i8 [ 3, %sw.default ], [ 44, %sw.bb3 ], [ -56, %sw.bb2 ], [ 100, %sw.bb1 ], [ 0, %entry ]
ret i8 %x
; CHECK-LABEL: @linearmap3(
; CHECK: entry:
; CHECK-NEXT: %switch.tableidx = sub i32 %c, 10
; CHECK: switch.lookup:
; CHECK-NEXT: %switch.idx.cast = trunc i32 %switch.tableidx to i8
; CHECK-NEXT: %switch.idx.mult = mul i8 %switch.idx.cast, 100
; CHECK-NEXT: ret i8 %switch.idx.mult
}
; Linear mapping with with multiplier 1 and offset 0.
define i8 @linearmap4(i32 %c) {
entry:
switch i32 %c, label %sw.default [
i32 -2, label %return
i32 -1, label %sw.bb1
i32 0, label %sw.bb2
i32 1, label %sw.bb3
]
sw.bb1: br label %return
sw.bb2: br label %return
sw.bb3: br label %return
sw.default: br label %return
return:
%x = phi i8 [ 3, %sw.default ], [ 3, %sw.bb3 ], [ 2, %sw.bb2 ], [ 1, %sw.bb1 ], [ 0, %entry ]
ret i8 %x
; CHECK-LABEL: @linearmap4(
; CHECK: entry:
; CHECK-NEXT: %switch.tableidx = sub i32 %c, -2
; CHECK: switch.lookup:
; CHECK-NEXT: %switch.idx.cast = trunc i32 %switch.tableidx to i8
; CHECK-NEXT: ret i8 %switch.idx.cast
}