[NVPTX] enable NaryReassociate in NVPTX

[jingyue]

Summary

We run NaryReassociate right after SLSR because SLSR enables many
opportunities for NaryReassociate. For example, in nary-slsr.ll

foo((a + b) + c);
foo((a + b * 2) + c);
foo((a + b * 3) + c);   // 2 muls and 6 adds

after SLSR:

ab = a + b;
foo(ab + c);
ab2 = ab + b;
foo(ab2 + c);
ab3 = ab2 + b;
foo(ab3 + c);           // 6 adds

after NaryReassociate:

abc = (a + b) + c;
foo(abc);
ab2c = abc + b;
foo(ab2c);
ab3c = ab2c + b;
foo(ab3c);              // 4 adds

[eliben]

LGTM

[jingyue]

rebase to trunk

lib/Target/NVPTX/NVPTXTargetMachine.cpp

void NVPTXPassConfig::addIRPasses() {
   addPass(createStraightLineStrengthReducePass());
   // SeparateConstOffsetFromGEP and SLSR creates common expressions which GVN or
   // EarlyCSE can reuse. GVN generates significantly better code than EarlyCSE
   // for some of our benchmarks.
   if (getOptLevel() == CodeGenOpt::Aggressive)
     addPass(createGVNPass());
   else
     addPass(createEarlyCSEPass());
+  // Run NaryReassociate after EarlyCSE/GVN to be more effective.
+  addPass(createNaryReassociatePass());
 }
 
 bool NVPTXPassConfig::addInstSelector() {
   const NVPTXSubtarget &ST = *getTM<NVPTXTargetMachine>().getSubtargetImpl();
 
   addPass(createLowerAggrCopies());
   addPass(createAllocaHoisting());
   addPass(createNVPTXISelDag(getNVPTXTargetMachine(), getOptLevel()));

test/Transforms/NaryReassociate/NVPTX/lit.local.cfg

+if not 'NVPTX' in config.root.targets:
+    config.unsupported = True

test/Transforms/NaryReassociate/NVPTX/nary-slsr.ll

+; RUN: opt < %s -slsr -nary-reassociate -S | FileCheck %s
+; RUN: llc < %s -march=nvptx64 -mcpu=sm_20 | FileCheck %s --check-prefix=PTX
+
+target datalayout = "e-i64:64-v16:16-v32:32-n16:32:64"
+
+; foo((a + b) + c);
+; foo((a + b * 2) + c);
+; foo((a + b * 3) + c);
+;   =>
+; abc = (a + b) + c;
+; foo(abc);
+; ab2c = abc + b;
+; foo(ab2c);
+; ab3c = ab2c + b;
+; foo(ab3c);
+define void @nary_reassociate_after_slsr(i32 %a, i32 %b, i32 %c) {
+; CHECK-LABEL: @nary_reassociate_after_slsr(
+; PTX-LABEL: .visible .func nary_reassociate_after_slsr(
+; PTX: ld.param.u32 [[b:%r[0-9]+]], [nary_reassociate_after_slsr_param_1];
+  %ab = add i32 %a, %b
+  %abc = add i32 %ab, %c
+  call void @foo(i32 %abc)
+; CHECK: call void @foo(i32 %abc)
+; PTX: st.param.b32 [param0+0], [[abc:%r[0-9]+]];
+
+  %b2 = shl i32 %b, 1
+  %ab2 = add i32 %a, %b2
+  %ab2c = add i32 %ab2, %c
+; CHECK-NEXT: %ab2c = add i32 %abc, %b
+; PTX: add.s32 [[ab2c:%r[0-9]+]], [[abc]], [[b]]
+  call void @foo(i32 %ab2c)
+; CHECK-NEXT: call void @foo(i32 %ab2c)
+; PTX: st.param.b32 [param0+0], [[ab2c]];
+
+  %b3 = mul i32 %b, 3
+  %ab3 = add i32 %a, %b3
+  %ab3c = add i32 %ab3, %c
+; CHECK-NEXT: %ab3c = add i32 %ab2c, %b
+; PTX: add.s32 [[ab3c:%r[0-9]+]], [[ab2c]], [[b]]
+  call void @foo(i32 %ab3c)
+; CHECK-NEXT: call void @foo(i32 %ab3c)
+; PTX: st.param.b32 [param0+0], [[ab3c]];
+
+  ret void
+}
+
+declare void @foo(i32)