[clang] [llvm] [AVR] Fix Avr indvar detection and strength reduction (missed optimization) (PR #152028)

[Tom Vijlbrief via llvm-commits]

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@@ -0,0 +1,95 @@
+; NOTE: Assertions have been autogenerated by utils/update_llc_test_checks.py
+; RUN: llc < %s -O=3 -mtriple=avr-none -mcpu=attiny85 -verify-machineinstrs | FileCheck %s
+
+declare dso_local void @foo(i16 noundef) addrspace(1)
+ at ci = dso_local global [30 x i16] zeroinitializer, align 1
+define void @loopreduce() {
+; CHECK-LABEL: loopreduce:
+; CHECK:       ; %bb.0: ; %entry
+; CHECK-NEXT:    push r14
+; CHECK-NEXT:    push r15
+; CHECK-NEXT:    push r16
+; CHECK-NEXT:    push r17
+; CHECK-NEXT:    ldi r26, lo8(ci)
+; CHECK-NEXT:    ldi r27, hi8(ci)
+; CHECK-NEXT:    ldi r16, lo8(ci+60)
+; CHECK-NEXT:    ldi r17, hi8(ci+60)
+; CHECK-NEXT:  .LBB0_1: ; %for.body
+; CHECK-NEXT:    ; =>This Inner Loop Header: Depth=1
+; CHECK-NEXT:    ld r24, X+
+; CHECK-NEXT:    ld r25, X+
+; CHECK-NEXT:    movw r14, r26
+; CHECK-NEXT:    rcall foo
+; CHECK-NEXT:    movw r26, r14
+; CHECK-NEXT:    cp r26, r16
+; CHECK-NEXT:    cpc r27, r17
+; CHECK-NEXT:    brne .LBB0_1
+; CHECK-NEXT:  ; %bb.2: ; %for.cond.cleanup
+; CHECK-NEXT:    pop r17
+; CHECK-NEXT:    pop r16
+; CHECK-NEXT:    pop r15
+; CHECK-NEXT:    pop r14
+; CHECK-NEXT:    ret
+entry:
+  br label %for.body
+for.body:                                         ; preds = %entry, %for.body
+  %i.03 = phi i16 [ 0, %entry ], [ %inc, %for.body ]
+  %arrayidx = getelementptr inbounds nuw [30 x i16], ptr @ci, i16 0, i16 %i.03
+  %0 = load i16, ptr %arrayidx, align 1
+  tail call addrspace(1) void @foo(i16 noundef %0)
+  %inc = add nuw nsw i16 %i.03, 1
+  %exitcond.not = icmp eq i16 %inc, 30
+  br i1 %exitcond.not, label %for.cond.cleanup, label %for.body
+; Exit blocks
+for.cond.cleanup:                                 ; preds = %for.body
+  ret void
+}
+
+define void @indvar() {
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tomtor wrote:

There is a minimal difference, loop starting at 7 vs 8 and different ordering... I would like to understand why we don't get the optimization in LLC. I leave the test in as reminder.

https://github.com/llvm/llvm-project/pull/152028