[LoopRotate] Don't perform loop rotation if the loop header calls a convergent function.

[jlebar]

Summary

Calls to convergent functions can be duplicated, but only if the
duplicates are not control-flow dependent on any additional values.
Loop rotation doesn't meet the bar.

[jingyue]

test/Transforms/LoopRotate/convergent.ll

16 ↗

(On Diff #47595)

Nit: Can we simplify the test by passing %end as a parameter? That gets rid of @e and the load instruction.

llvm/trunk/lib/Transforms/Scalar/LoopRotation.cpp

// duplicate blocks inside it.
 
     CodeMetrics Metrics;
     Metrics.analyzeBasicBlock(OrigHeader, *TTI, EphValues);
     if (Metrics.notDuplicatable) {
       DEBUG(dbgs() << "LoopRotation: NOT rotating - contains non-duplicatable"
             << " instructions: "; L->dump());
       return false;
     }
+    if (Metrics.convergent) {
+      DEBUG(dbgs() << "LoopRotation: NOT rotating - contains convergent "
+                      "instructions: "; L->dump());
+      return false;
+    }
     if (Metrics.NumInsts > MaxHeaderSize)
       return false;
   }
 
   // Now, this loop is suitable for rotation.
   BasicBlock *OrigPreheader = L->getLoopPreheader();
 
   // If the loop could not be converted to canonical form, it must have an

llvm/trunk/test/Transforms/LoopRotate/convergent.ll

+; RUN: opt -S -loop-rotate < %s | FileCheck %s
+
+@e = global i32 10
+
+declare void @f1(i32) convergent
+declare void @f2(i32)
+
+; The call to f1 in the loop header shouldn't be duplicated (meaning, loop
+; rotation shouldn't occur), because f1 is convergent.
+
+; CHECK: call void @f1
+; CHECK-NOT: call void @f1
+
+define void @test(i32 %x) {
+entry:
+  br label %loop
+
+loop:
+  %n.phi = phi i32 [ %n, %loop.fin ], [ 0, %entry ]
+  call void @f1(i32 %n.phi)
+  %cond = icmp eq i32 %n.phi, %x
+  br i1 %cond, label %exit, label %loop.fin
+
+loop.fin:
+  %n = add i32 %n.phi, 1
+  call void @f2(i32 %n)
+  br label %loop
+
+exit:
+  ret void
+}