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[ARM] Allow rematerialization of ARM Thumb literal pool loads

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Constants are crucial for code size in the ARM Thumb-1 instruction
set. The 16 bit instruction size often does not offer enough space
for immediate arguments. This means that additional instructions are
frequently used to load constants into registers. Since constants are
hoisted, this can lead to significant register spillage if they are
used multiple times in a single function. This can be avoided by
rematerialization, i.e. recomputing a constant instead of reloading
it from the stack. This patch fixes the rematerialization of literal
pool loads in the ARM Thumb instruction set.

Patch by Philip Ginsbach

Differential Revision: https://reviews.llvm.org/D33936


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@308004 91177308-0d34-0410-b5e6-96231b3b80d8
This commit is contained in:
Sam Parker 2017-07-14 08:23:56 +00:00
parent a9a5cb971f
commit 676084e2a9
3 changed files with 47 additions and 5 deletions
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20
lib/Target/ARM/ARMISelDAGToDAG.cpp
View File

@ -2682,9 +2682,12 @@ void ARMDAGToDAGISel::Select(SDNode *N) {
SDNode *ResNode;
if (Subtarget->isThumb()) {
SDValue Pred = getAL(CurDAG, dl);
SDValue PredReg = CurDAG->getRegister(0, MVT::i32);
SDValue Ops[] = { CPIdx, Pred, PredReg, CurDAG->getEntryNode() };
SDValue Ops[] = {
CPIdx,
getAL(CurDAG, dl),
CurDAG->getRegister(0, MVT::i32),
CurDAG->getEntryNode()
};
ResNode = CurDAG->getMachineNode(ARM::tLDRpci, dl, MVT::i32, MVT::Other,
Ops);
} else {
@ -2698,6 +2701,17 @@ void ARMDAGToDAGISel::Select(SDNode *N) {
ResNode = CurDAG->getMachineNode(ARM::LDRcp, dl, MVT::i32, MVT::Other,
Ops);
}
// Annotate the Node with memory operand information so that MachineInstr
// queries work properly. This e.g. gives the register allocation the
// required information for rematerialization.
MachineFunction& MF = CurDAG->getMachineFunction();
MachineSDNode::mmo_iterator MemOp = MF.allocateMemRefsArray(1);
MemOp[0] = MF.getMachineMemOperand(
MachinePointerInfo::getConstantPool(MF),
MachineMemOperand::MOLoad, 4, 4);
cast<MachineSDNode>(ResNode)->setMemRefs(MemOp, MemOp+1);
ReplaceNode(N, ResNode);
return;
}

28
test/CodeGen/Thumb/litpoolremat.ll Normal file
View File

@ -0,0 +1,28 @@
; RUN: llc < %s -mtriple=thumb-apple-darwin | FileCheck %s
declare void @consume_value(i32) #1
declare i32 @get_value(...) #1
declare void @consume_three_values(i32, i32, i32) #1
; Function Attrs: nounwind uwtable
define void @should_not_spill() #0 {
tail call void @consume_value(i32 1764) #2
%1 = tail call i32 (...) @get_value() #2
%2 = tail call i32 (...) @get_value() #2
%3 = tail call i32 (...) @get_value() #2
tail call void @consume_value(i32 %1) #2
tail call void @consume_value(i32 %2) #2
tail call void @consume_value(i32 %3) #2
tail call void @consume_value(i32 1764) #2
tail call void @consume_three_values(i32 %1, i32 %2, i32 %3) #2
ret void
}
; CHECK: ldr r0, LCPI0_0
; CHECK-NOT: str r0
; CHECK: bl
; CHECK: ldr r0, LCPI0_0
; CHECK-LABEL: LCPI0_0:
; CHECK-NEXT: .long 1764

4
test/CodeGen/Thumb/select.ll
View File

@ -74,9 +74,9 @@ define double @f7(double %a, double %b) {
}
; CHECK-LABEL: f7:
; CHECK: blt
; CHECK: blt
; CHECK: {{blt|bge}}
; CHECK: __ltdf2
; CHECK-EABI-LABEL: f7:
; CHECK-EABI: __aeabi_dcmplt
; CHECK-EABI: bne
; CHECK-EABI: bne
; CHECK-EABI: {{bne|beq}}