[llvm] [AMDGPU][TTI] Threshold bonus to loops whose unrolling makes nested loops unrollable (PR #114579)

[Matt Arsenault via llvm-commits]

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+; RUN: opt -S -mtriple=amdgcn-- -passes=loop-unroll -debug-only=AMDGPUtti < %s 2>&1 | FileCheck %s
+
+; For @dependent_sub_fullunroll, the threshold bonus should apply
+; CHECK: due to subloop's trip count becoming runtime-independent after unrolling
+
+; For @dependent_sub_no_fullunroll, the threshold bonus should not apply
+; CHECK-NOT: due to subloop's trip count becoming runtime-independent after unrolling
+
+; Check that the outer loop of a double-nested loop where the inner loop's trip
+; count depends exclusively on constants and the outer IV is fully unrolled
+; thanks to receiving a threshold bonus in AMDGPU's TTI.
+
+; CHECK-LABEL: @dependent_sub_fullunroll
+; CHECK: inner.header_latch_exiting.7
+; CHECK: outer.latch_exiting.7
+
+define void @dependent_sub_fullunroll(ptr noundef %mem) {
+entry:
+  br label %outer.header
+
+outer.header:                                                 ; preds = %entry, %outer.latch_exiting
+  %outer.iv = phi i32 [ 0, %entry ], [ %outer.iv_next, %outer.latch_exiting ]
+  br label %inner.header_latch_exiting
+
+inner.header_latch_exiting:                                   ; preds = %outer.header, %inner.header_latch_exiting
+  %inner.iv = phi i32 [ %outer.iv, %outer.header ], [ %inner.iv_next, %inner.header_latch_exiting ]
+  %inner.iv_next = add nuw nsw i32 %inner.iv, 1
+  %outer.iv.ext = zext nneg i32 %outer.iv to i64
+  %idx_part = mul nuw nsw i64 %outer.iv.ext, 16 
+  %inner.iv.ext = zext nneg i32 %inner.iv to i64
+  %idx = add nuw nsw i64 %idx_part, %inner.iv.ext 
+  %addr = getelementptr inbounds i8, ptr %mem, i64 %idx
+  store i32 0, ptr %addr
+  %inner.cond = icmp ult i32 %inner.iv_next, 8
+  br i1 %inner.cond, label %inner.header_latch_exiting, label %outer.latch_exiting, !llvm.loop !1
+
+outer.latch_exiting:                                          ; preds = %inner.header_latch_exiting
+  %outer.iv_next = add nuw nsw i32 %outer.iv, 1
+  %outer.cond = icmp ult i32 %outer.iv_next, 8
+  br i1 %outer.cond, label %outer.header, label %end, !llvm.loop !1
+  
+end:                                                          ; preds = %outer.latch_exiting
+  ret void
+}
+
+; Check that the outer loop of the same loop nest as dependent_sub_fullunroll
+; is not fully unrolled when the inner loop's final IV value depends on a
+; function argument instead of a combination of the outer IV and constants.
+
+; CHECK-LABEL: @dependent_sub_no_fullunroll
+; CHECK-NOT: outer.latch_exiting.7
+; CHECK-NOT: outer.latch_exiting.7
+
+define void @dependent_sub_no_fullunroll(ptr noundef %mem, i32 noundef %inner.ub) {
+entry:
+  br label %outer.header
+
+outer.header:                                                 ; preds = %entry, %outer.latch_exiting
+  %outer.iv = phi i32 [ 0, %entry ], [ %outer.iv_next, %outer.latch_exiting ]
+  br label %inner.header_latch_exiting
+
+inner.header_latch_exiting:                                   ; preds = %outer.header, %inner.header_latch_exiting
+  %inner.iv = phi i32 [ %outer.iv, %outer.header ], [ %inner.iv_next, %inner.header_latch_exiting ]
+  %inner.iv_next = add nuw nsw i32 %inner.iv, 1
+  %outer.iv.ext = zext nneg i32 %outer.iv to i64
+  %idx_part = mul nuw nsw i64 %outer.iv.ext, 16 
+  %inner.iv.ext = zext nneg i32 %inner.iv to i64
+  %idx = add nuw nsw i64 %idx_part, %inner.iv.ext 
+  %addr = getelementptr inbounds i8, ptr %mem, i64 %idx
+  store i32 0, ptr %addr
+  %inner.cond = icmp ult i32 %inner.iv_next, %inner.ub
+  br i1 %inner.cond, label %inner.header_latch_exiting, label %outer.latch_exiting, !llvm.loop !1
+
+outer.latch_exiting:                                          ; preds = %inner.header_latch_exiting
+  %outer.iv_next = add nuw nsw i32 %outer.iv, 1
+  %outer.cond = icmp ult i32 %outer.iv_next, 8
+  br i1 %outer.cond, label %outer.header, label %end, !llvm.loop !1
+  
+end:                                                          ; preds = %outer.latch_exiting
+  ret void
+}
+
----------------
arsenm wrote:

Can you add a case with a convergent operation that would be illegal to unroll, to make sure this doesn't accidentally override a correctness check 

https://github.com/llvm/llvm-project/pull/114579