[ARM] Restore hasSideEffects flag on t2WhileLoopSetup (#168948)

ARM relies on deprecated TableGen behavior of guessing instruction
properties from patterns (`def ARM : Target` doesn't have
`guessInstructionProperties` set to false).

Before #168209, TableGen conservatively guessed that `t2WhileLoopSetup`
has side effects because the instruction wasn't matched by any pattern.

After the patch, TableGen guesses it has no side effects because the
added pattern uses only `arm_wlssetup` node, which has no side effects.

Add `SDNPSideEffect` to the node so that TableGen guesses the property
right, and also `hasSideEffects = 1` to the instruction in case ARM ever
sets `guessInstructionProperties` to false.

2 files changed, 48 insertions, 1 deletions

diff --git a/llvm/lib/Target/ARM/ARMInstrThumb2.td b/llvm/lib/Target/ARM/ARMInstrThumb2.td
index 317959c0342f..66a2297bde0b 100644
--- a/llvm/lib/Target/ARM/ARMInstrThumb2.td
+++ b/llvm/lib/Target/ARM/ARMInstrThumb2.td
@@ -5584,7 +5584,8 @@ class t2LOL<dag oops, dag iops, string asm, string ops>
 // Setup for the iteration count of a WLS. See t2WhileLoopSetup.
 def arm_wlssetup
     : SDNode<"ARMISD::WLSSETUP",
-             SDTypeProfile<1, 1, [SDTCisInt<0>, SDTCisSameAs<1, 0>]>>;
+             SDTypeProfile<1, 1, [SDTCisInt<0>, SDTCisSameAs<1, 0>]>,
+             [SDNPSideEffect]>;
 
 // Low-overhead loops, While Loop Start branch. See t2WhileLoopStart
 def arm_wls : SDNode<"ARMISD::WLS",
@@ -5668,6 +5669,7 @@ def t2DoLoopStartTP :
 // t2WhileLoopSetup to setup LR and t2WhileLoopStart to perform the branch. Not
 // valid after reg alloc, as it should be lowered during MVETPAndVPTOptimisations
 // into a t2WhileLoopStartLR (or expanded).
+let hasSideEffects = 1 in
 def t2WhileLoopSetup :
     t2PseudoInst<(outs GPRlr:$lr), (ins rGPR:$tc), 4, IIC_Br,
                  [(set i32:$lr, (arm_wlssetup i32:$tc))]>;
diff --git a/llvm/test/CodeGen/Thumb2/LowOverheadLoops/pr168209.ll b/llvm/test/CodeGen/Thumb2/LowOverheadLoops/pr168209.ll
new file mode 100644
index 000000000000..a6dded12c064
--- /dev/null
+++ b/llvm/test/CodeGen/Thumb2/LowOverheadLoops/pr168209.ll
@@ -0,0 +1,45 @@
+; NOTE: Assertions have been autogenerated by utils/update_llc_test_checks.py UTC_ARGS: --version 6
+; RUN: llc -mtriple=thumbv8.1m.main %s -o - | FileCheck %s
+
+; Checks that t2WhileLoopSetup is not CSEd.
+
+define i32 @test(i16 %arg) {
+; CHECK-LABEL: test:
+; CHECK:       @ %bb.0: @ %bb
+; CHECK-NEXT:    push {r7, lr}
+; CHECK-NEXT:    uxth r0, r0
+; CHECK-NEXT:    wls lr, r0, .LBB0_4
+; CHECK-NEXT:  .LBB0_1: @ %bb3
+; CHECK-NEXT:    @ =>This Inner Loop Header: Depth=1
+; CHECK-NEXT:    le lr, .LBB0_1
+; CHECK-NEXT:  @ %bb.2: @ %bb2
+; CHECK-NEXT:    wls lr, r0, .LBB0_4
+; CHECK-NEXT:  .LBB0_3: @ %bb7
+; CHECK-NEXT:    @ =>This Inner Loop Header: Depth=1
+; CHECK-NEXT:    le lr, .LBB0_3
+; CHECK-NEXT:  .LBB0_4: @ %.critedge
+; CHECK-NEXT:    movs r0, #0
+; CHECK-NEXT:    pop {r7, pc}
+bb:
+  %i = zext i16 %arg to i32
+  %i1 = icmp eq i16 %arg, 0
+  br i1 %i1, label %.critedge, label %bb3
+
+bb2:                                              ; preds = %bb3
+  br i1 %i1, label %.critedge, label %bb7
+
+bb3:                                              ; preds = %bb3, %bb
+  %i4 = phi i32 [ %i5, %bb3 ], [ 0, %bb ]
+  %i5 = add i32 %i4, 1
+  %i6 = icmp eq i32 %i5, %i
+  br i1 %i6, label %bb2, label %bb3
+
+bb7:                                              ; preds = %bb7, %bb2
+  %i8 = phi i32 [ %i9, %bb7 ], [ 0, %bb2 ]
+  %i9 = add i32 %i8, 1
+  %i10 = icmp eq i32 %i9, %i
+  br i1 %i10, label %.critedge, label %bb7
+
+.critedge:                                        ; preds = %bb7, %bb2, %bb
+  ret i32 0
+}