[ARM] Deliberately prevent inline asm in low overhead loops. NFC

[dmgreen]

Summary

This was already something that was handled by one of the "else" branches in maybeLoweredToCall, so this patch is an NFC but makes it explicit and adds a test. We may in the future want to support this under certain situations but for the moment just don't try and create low overhead loops with inline asm in them.

[samparker]

Explicit sounds good.

llvm/lib/Target/ARM/ARMTargetTransformInfo.cpp

auto IsHardwareLoopIntrinsic = [](Instruction &I) {
     return false;
   };
 
   // Scan the instructions to see if there's any that we know will turn into a
   // call or if this loop is already a low-overhead loop.
   auto ScanLoop = [&](Loop *L) {
     for (auto *BB : L->getBlocks()) {
       for (auto &I : *BB) {
-        if (maybeLoweredToCall(I) || IsHardwareLoopIntrinsic(I)) {
+        if (maybeLoweredToCall(I) || IsHardwareLoopIntrinsic(I) ||
+            isa<InlineAsm>(I)) {
           LLVM_DEBUG(dbgs() << "ARMHWLoops: Bad instruction: " << I << "\n");
           return false;
         }
       }
     }
     return true;
   };
 

llvm/test/CodeGen/Thumb2/LowOverheadLoops/inlineasm.ll

+; NOTE: Assertions have been autogenerated by utils/update_llc_test_checks.py
+; RUN: llc -mtriple=thumbv8.1m.main-none-none-eabi -mattr=+mve -verify-machineinstrs -o - %s | FileCheck %s
+
+define i32 @test(i16* nocapture readonly %x, i16* nocapture readonly %y, i32 %n) {
+; CHECK-LABEL: test:
+; CHECK:       @ %bb.0: @ %entry
+; CHECK-NEXT:    .save {r7, lr}
+; CHECK-NEXT:    push {r7, lr}
+; CHECK-NEXT:    cmp r2, #1
+; CHECK-NEXT:    blt .LBB0_4
+; CHECK-NEXT:  @ %bb.1: @ %for.body.preheader
+; CHECK-NEXT:    mov lr, r0
+; CHECK-NEXT:    movs r0, #0
+; CHECK-NEXT:  .LBB0_2: @ %for.body
+; CHECK-NEXT:    @ =>This Inner Loop Header: Depth=1
+; CHECK-NEXT:    ldrh r3, [r1], #2
+; CHECK-NEXT:    subs r2, #1
+; CHECK-NEXT:    ldrh r12, [lr], #2
+; CHECK-NEXT:    @APP
+; CHECK-NEXT:    add r3, r12
+; CHECK-NEXT:    @NO_APP
+; CHECK-NEXT:    add r0, r3
+; CHECK-NEXT:    bne .LBB0_2
+; CHECK-NEXT:  @ %bb.3: @ %for.cond.cleanup
+; CHECK-NEXT:    pop {r7, pc}
+; CHECK-NEXT:  .LBB0_4:
+; CHECK-NEXT:    movs r0, #0
+; CHECK-NEXT:    pop {r7, pc}
+entry:
+  %cmp9 = icmp sgt i32 %n, 0
+  br i1 %cmp9, label %for.body, label %for.cond.cleanup
+
+for.cond.cleanup:                                 ; preds = %for.body, %entry
+  %s.0.lcssa = phi i32 [ 0, %entry ], [ %add, %for.body ]
+  ret i32 %s.0.lcssa
+
+for.body:                                         ; preds = %entry, %for.body
+  %s.011 = phi i32 [ %add, %for.body ], [ 0, %entry ]
+  %i.010 = phi i32 [ %inc, %for.body ], [ 0, %entry ]
+  %arrayidx = getelementptr inbounds i16, i16* %x, i32 %i.010
+  %0 = load i16, i16* %arrayidx, align 2
+  %arrayidx1 = getelementptr inbounds i16, i16* %y, i32 %i.010
+  %1 = load i16, i16* %arrayidx1, align 2
+  %2 = tail call i32 asm "add $0, $1, $2", "=r,r,r"(i16 %0, i16 %1) #1
+  %add = add nsw i32 %2, %s.011
+  %inc = add nuw nsw i32 %i.010, 1
+  %exitcond.not = icmp eq i32 %inc, %n
+  br i1 %exitcond.not, label %for.cond.cleanup, label %for.body
+}
+
+define i32 @testlr(i16* nocapture readonly %x, i16* nocapture readonly %y, i32 %n) {
+; CHECK-LABEL: testlr:
+; CHECK:       @ %bb.0: @ %entry
+; CHECK-NEXT:    .save {r4, lr}
+; CHECK-NEXT:    push {r4, lr}
+; CHECK-NEXT:    cmp r2, #1
+; CHECK-NEXT:    blt .LBB1_4
+; CHECK-NEXT:  @ %bb.1: @ %for.body.preheader
+; CHECK-NEXT:    mov r3, r0
+; CHECK-NEXT:    movs r0, #0
+; CHECK-NEXT:  .LBB1_2: @ %for.body
+; CHECK-NEXT:    @ =>This Inner Loop Header: Depth=1
+; CHECK-NEXT:    ldrh r4, [r1], #2
+; CHECK-NEXT:    subs r2, #1
+; CHECK-NEXT:    ldrh r12, [r3], #2
+; CHECK-NEXT:    @APP
+; CHECK-NEXT:    add r4, r12
+; CHECK-NEXT:    @NO_APP
+; CHECK-NEXT:    add r0, r4
+; CHECK-NEXT:    bne .LBB1_2
+; CHECK-NEXT:  @ %bb.3: @ %for.cond.cleanup
+; CHECK-NEXT:    pop {r4, pc}
+; CHECK-NEXT:  .LBB1_4:
+; CHECK-NEXT:    movs r0, #0
+; CHECK-NEXT:    pop {r4, pc}
+entry:
+  %cmp9 = icmp sgt i32 %n, 0
+  br i1 %cmp9, label %for.body, label %for.cond.cleanup
+
+for.cond.cleanup:                                 ; preds = %for.body, %entry
+  %s.0.lcssa = phi i32 [ 0, %entry ], [ %add, %for.body ]
+  ret i32 %s.0.lcssa
+
+for.body:                                         ; preds = %entry, %for.body
+  %s.011 = phi i32 [ %add, %for.body ], [ 0, %entry ]
+  %i.010 = phi i32 [ %inc, %for.body ], [ 0, %entry ]
+  %arrayidx = getelementptr inbounds i16, i16* %x, i32 %i.010
+  %0 = load i16, i16* %arrayidx, align 2
+  %arrayidx1 = getelementptr inbounds i16, i16* %y, i32 %i.010
+  %1 = load i16, i16* %arrayidx1, align 2
+  %2 = tail call i32 asm "add $0, $1, $2", "=r,r,r,~{lr}"(i16 %0, i16 %1) #1
+  %add = add nsw i32 %2, %s.011
+  %inc = add nuw nsw i32 %i.010, 1
+  %exitcond.not = icmp eq i32 %inc, %n
+  br i1 %exitcond.not, label %for.cond.cleanup, label %for.body
+}