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
diff --git a/lib/Target/ARM/ARMISelLowering.cpp b/lib/Target/ARM/ARMISelLowering.cpp
index e3c6273..7e2bbcd 100644
--- a/lib/Target/ARM/ARMISelLowering.cpp
+++ b/lib/Target/ARM/ARMISelLowering.cpp
@@ -402,6 +402,7 @@
case ARMISD::tCALL: return "ARMISD::tCALL";
case ARMISD::BRCOND: return "ARMISD::BRCOND";
case ARMISD::BR_JT: return "ARMISD::BR_JT";
+ case ARMISD::BR2_JT: return "ARMISD::BR2_JT";
case ARMISD::RET_FLAG: return "ARMISD::RET_FLAG";
case ARMISD::PIC_ADD: return "ARMISD::PIC_ADD";
case ARMISD::CMP: return "ARMISD::CMP";
@@ -1704,15 +1705,27 @@
SDValue UId = DAG.getConstant(AFI->createJumpTableUId(), PTy);
SDValue JTI = DAG.getTargetJumpTable(JT->getIndex(), PTy);
Table = DAG.getNode(ARMISD::WrapperJT, dl, MVT::i32, JTI, UId);
+ if (Subtarget->isThumb2()) {
+ // Thumb2 uses a two-level jump. That is, it jumps into the jump table
+ // which does another jump to the destination. This also makes it easier
+ // to translate it to TBB / TBH later.
+ // FIXME: This might not work if the function is extremely large.
+ return DAG.getNode(ARMISD::BR2_JT, dl, MVT::Other, Chain, Table, Index,
+ JTI, UId);
+ }
+
Index = DAG.getNode(ISD::MUL, dl, PTy, Index, DAG.getConstant(4, PTy));
SDValue Addr = DAG.getNode(ISD::ADD, dl, PTy, Index, Table);
- bool isPIC = getTargetMachine().getRelocationModel() == Reloc::PIC_;
- Addr = DAG.getLoad(isPIC ? (MVT)MVT::i32 : PTy, dl,
- Chain, Addr, NULL, 0);
- Chain = Addr.getValue(1);
- if (isPIC)
+ if (getTargetMachine().getRelocationModel() == Reloc::PIC_) {
+ Addr = DAG.getLoad((MVT)MVT::i32, dl, Chain, Addr, NULL, 0);
+ Chain = Addr.getValue(1);
Addr = DAG.getNode(ISD::ADD, dl, PTy, Addr, Table);
- return DAG.getNode(ARMISD::BR_JT, dl, MVT::Other, Chain, Addr, JTI, UId);
+ return DAG.getNode(ARMISD::BR_JT, dl, MVT::Other, Chain, Addr, JTI, UId);
+ } else {
+ Addr = DAG.getLoad(PTy, dl, Chain, Addr, NULL, 0);
+ Chain = Addr.getValue(1);
+ return DAG.getNode(ARMISD::BR_JT, dl, MVT::Other, Chain, Addr, JTI, UId);
+ }
}
static SDValue LowerFP_TO_INT(SDValue Op, SelectionDAG &DAG) {