[LSR] If no Use is interesting, early return.

[timshen]

Summary

By looking at the callers of getUse(), we can see that even though
IVUsers may offer uses, but they may not be interesting to
LSR. It's possible that none of them is interesting.

[sanjoy]

Do you have a test case for this?

[timshen]

Added test case.

[timshen]

In D49049#1154974, @sanjoy wrote:

Do you have a test case for this?

I added the crasher. I haven't take a close look at the IR, but it's sufficient to reproduce the crash. Let me know if you think it's helpful to fully study the test and justify the cause.

[sanjoy]

I'd debug this a bit further, perhaps with some logging statements to figure out what happened (what use did we discard as non-interesting and why?). But if that looks like too much work then this change as-is is fine too.

llvm/test/CodeGen/X86/lsr-crash-empty-uses.ll

2

Can't you run LSR directly using opt?

40

Would be nice to clean up these names. Maybe use opt -metarenamer?

[timshen]

TIL -metarenamer.

llvm/test/CodeGen/X86/lsr-crash-empty-uses.ll

2

It's weird. When I switch to opt it doesn't crash.

[timshen]

llvm/test/CodeGen/X86/lsr-crash-empty-uses.ll

2

LSR has lots of target-specific logic, so maybe it makes sense to use llc in this case.

llvm/lib/Transforms/Scalar/LoopStrengthReduce.cpp

#endif // DEBUG
   }
 
   // Start collecting data and preparing for the solver.
   CollectChains();
   CollectInterestingTypesAndFactors();
   CollectFixupsAndInitialFormulae();
   CollectLoopInvariantFixupsAndFormulae();
 
-  assert(!Uses.empty() && "IVUsers reported at least one use");
+  if (Uses.empty())
+    return;
+
   LLVM_DEBUG(dbgs() << "LSR found " << Uses.size() << " uses:\n";
              print_uses(dbgs()));
 
   // Now use the reuse data to generate a bunch of interesting ways
   // to formulate the values needed for the uses.
   GenerateAllReuseFormulae();
 
   FilterOutUndesirableDedicatedRegisters();

llvm/test/CodeGen/X86/lsr-crash-empty-uses.ll

+; RUN: llc --start-before loop-reduce --stop-after loop-reduce %s -o - | FileCheck %s
+target datalayout = "e-m:e-p:32:32-i64:64-n32-S128"
+target triple = "x86_64-unknown-linux-gnu"
+
+define void @hoge() {
+bb:
+  %tmp = sext i32 undef to i64
+  %tmp3 = sub nsw i64 0, %tmp
+  br label %bb4
+
+bb4:                                              ; preds = %bb20, %bb
+  %tmp5 = getelementptr inbounds double, double* undef, i64 undef
+  %tmp6 = getelementptr inbounds double, double* %tmp5, i64 %tmp3
+  br label %bb7
+
+bb7:                                              ; preds = %bb7, %bb4
+  %tmp8 = phi double* [ %tmp10, %bb7 ], [ %tmp6, %bb4 ]
+  %tmp9 = load double, double* %tmp8
+  %tmp10 = getelementptr inbounds double, double* %tmp8, i64 1
+  br i1 true, label %bb11, label %bb7
+
+bb11:                                             ; preds = %bb7
+  br i1 undef, label %bb20, label %bb12
+
+bb12:                                             ; preds = %bb11
+  br label %bb13
+
+bb13:                                             ; preds = %bb13, %bb12
+  %tmp14 = phi double* [ %tmp18, %bb13 ], [ %tmp10, %bb12 ]
+  %tmp15 = load double, double* %tmp14, align 8
+  %tmp16 = getelementptr inbounds double, double* %tmp14, i64 1
+  %tmp17 = load double, double* %tmp16, align 8
+  %tmp18 = getelementptr inbounds double, double* %tmp14, i64 8
+  br i1 true, label %bb19, label %bb13
+
+bb19:                                             ; preds = %bb13
+  br label %bb20
+
+bb20:                                             ; preds = %bb19, %bb11
+  br label %bb4
+}