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Two case switch to select optimization

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This optimization tries to convert switch instructions that are used to select a value with only 2 unique cases + default block
to a select or a couple of selects (depending if the default block is reachable or not).

The typical case this optimization wants to be able to optimize is this one:

Example:
switch (a) {
  case 10:                %0 = icmp eq i32 %a, 10
    return 10;            %1 = select i1 %0, i32 10, i32 4
  case 20:        ---->   %2 = icmp eq i32 %a, 20
    return 2;             %3 = select i1 %2, i32 2, i32 %1
  default:
    return 4;
}

It also sets the base for further optimizations that are planned and being reviewed.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@219223 91177308-0d34-0410-b5e6-96231b3b80d8
This commit is contained in:
Marcello Maggioni
2014-10-07 18:16:44 +00:00
parent ebc756d0d7
commit 5c98f14b78
4 changed files with 249 additions and 3 deletions
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72
test/Transforms/SimplifyCFG/switch-to-select-two-case.ll Normal file
View File

@@ -0,0 +1,72 @@
; RUN: opt < %s -simplifycfg -S | FileCheck %s
; int foo1_with_default(int a) {
; switch(a) {
; case 10:
; return 10;
; case 20:
; return 2;
; }
; return 4;
; }
define i32 @foo1_with_default(i32 %a) {
; CHECK-LABEL: @foo1_with_default
; CHECK: %switch.selectcmp = icmp eq i32 %a, 20
; CHECK-NEXT: %switch.select = select i1 %switch.selectcmp, i32 2, i32 4
; CHECK-NEXT: %switch.selectcmp1 = icmp eq i32 %a, 10
; CHECK-NEXT: %switch.select2 = select i1 %switch.selectcmp1, i32 10, i32 %switch.select
entry:
switch i32 %a, label %sw.epilog [
i32 10, label %sw.bb
i32 20, label %sw.bb1
]
sw.bb:
br label %return
sw.bb1:
br label %return
sw.epilog:
br label %return
return:
%retval.0 = phi i32 [ 4, %sw.epilog ], [ 2, %sw.bb1 ], [ 10, %sw.bb ]
ret i32 %retval.0
}
; int foo1_without_default(int a) {
; switch(a) {
; case 10:
; return 10;
; case 20:
; return 2;
; }
; __builtin_unreachable();
; }
define i32 @foo1_without_default(i32 %a) {
; CHECK-LABEL: @foo1_without_default
; CHECK: %switch.selectcmp = icmp eq i32 %a, 10
; CHECK-NEXT: %switch.select = select i1 %switch.selectcmp, i32 10, i32 2
; CHECK-NOT: %switch.selectcmp1
entry:
switch i32 %a, label %sw.epilog [
i32 10, label %sw.bb
i32 20, label %sw.bb1
]
sw.bb:
br label %return
sw.bb1:
br label %return
sw.epilog:
unreachable
return:
%retval.0 = phi i32 [ 2, %sw.bb1 ], [ 10, %sw.bb ]
ret i32 %retval.0
}