Change Thumb2 jumptable codegen to one that uses two level jumps:

Before:
      adr r12, #LJTI3_0_0
      ldr pc, [r12, +r0, lsl #2]
LJTI3_0_0:
      .long    LBB3_24
      .long    LBB3_30
      .long    LBB3_31
      .long    LBB3_32

After:
      adr r12, #LJTI3_0_0
      add pc, r12, +r0, lsl #2
LJTI3_0_0:
      b.w    LBB3_24
      b.w    LBB3_30
      b.w    LBB3_31
      b.w    LBB3_32

This has several advantages.
1. This will make it easier to optimize this to a TBB / TBH instruction +
   (smaller) table.
2. This eliminate the need for ugly asm printer hack to force the address
   into thumb addresses (bit 0 is one).
3. Same codegen for pic and non-pic.
4. This eliminate the need to align the table so constantpool island pass
   won't have to over-estimate the size.

Based on my calculation, the later is probably slightly faster as well since
ldr pc with shifter address is very slow. That is, it should be a win as long
as the HW implementation can do a reasonable job of branch predict the second
branch.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@77024 91177308-0d34-0410-b5e6-96231b3b80d8

1 files changed, 5 insertions, 17 deletions

diff --git a/lib/Target/ARM/ARMInstrThumb2.td b/lib/Target/ARM/ARMInstrThumb2.td
index bb36a33..c265d9f 100644
--- a/lib/Target/ARM/ARMInstrThumb2.td
+++ b/lib/Target/ARM/ARMInstrThumb2.td
@@ -1080,24 +1080,12 @@ def t2B   : T2XI<(outs), (ins brtarget:$target),
                  "b $target",
                  [(br bb:$target)]>;
 
-let isNotDuplicable = 1, isIndirectBranch = 1 in {
-def t2BR_JTr : T2JTI<(outs), (ins GPR:$target, jtblock_operand:$jt, i32imm:$id),
-                     "mov pc, $target \n\t.align\t2\n$jt",
-                     [(ARMbrjt GPR:$target, tjumptable:$jt, imm:$id)]>;
-
-def t2BR_JTm : 
-    T2JTI<(outs),
-          (ins t2addrmode_so_reg:$target, jtblock_operand:$jt, i32imm:$id),
-          "ldr pc, $target \n\t.align\t2\n$jt",
-          [(ARMbrjt (i32 (load t2addrmode_so_reg:$target)), tjumptable:$jt,
-             imm:$id)]>;
-
-def t2BR_JTadd : 
+let isNotDuplicable = 1, isIndirectBranch = 1 in
+def t2BR_JT :
     T2JTI<(outs),
-          (ins GPR:$target, GPR:$idx, jtblock_operand:$jt, i32imm:$id),
-          "add pc, $target, $idx \n\t.align\t2\n$jt",
-          [(ARMbrjt (add GPR:$target, GPR:$idx), tjumptable:$jt, imm:$id)]>;
-} // isNotDuplicate, isIndirectBranch
+          (ins GPR:$base, GPR:$idx, jt2block_operand:$jt, i32imm:$id),
+          "add pc, $base, $idx, lsl #2\n$jt",
+          [(ARMbr2jt GPR:$base, GPR:$idx, tjumptable:$jt, imm:$id)]>;
 } // isBranch, isTerminator, isBarrier
 
 // FIXME: should be able to write a pattern for ARMBrcond, but can't use