[LICM/mustexec] Extend first iteration must execute logic to fcmps

This builds on the work from https://reviews.llvm.org/D44287. It turned out supporting fcmp was much easier than I realized, so let's do that now.

As an aside, our -O3 handling of a floating point IVs leaves a lot to be desired. We do convert the float IV to an integer IV, but do so late enough that many other optimizations are missed (e.g. we don't vectorize).

Differential Revision: https://reviews.llvm.org/D44542

llvm-svn: 327722

llvm/lib/Transforms/Utils/LoopUtils.cpp

@@ -1530,20 +1530,19 @@ static bool CanProveNotTakenFirstIteration(BasicBlock *ExitBlock,
   auto *BI = dyn_cast<BranchInst>(CondExitBlock->getTerminator());
   if (!BI || !BI->isConditional())
     return false;
-  // todo: handle fcmp someday
+  auto *Cond = dyn_cast<CmpInst>(BI->getCondition());
+  if (!Cond)
+    return false;
   // todo: this would be a lot more powerful if we used scev, but all the
   // plumbing is currently missing to pass a pointer in from the pass
-  auto *ICI = dyn_cast<ICmpInst>(BI->getCondition());
-  if (!ICI)
-    return false;
   // Check for cmp (phi [x, preheader] ...), y where (pred x, y is known
-  auto *LHS = dyn_cast<PHINode>(ICI->getOperand(0));
-  auto *RHS = ICI->getOperand(1);
+  auto *LHS = dyn_cast<PHINode>(Cond->getOperand(0));
+  auto *RHS = Cond->getOperand(1);
   if (!LHS || LHS->getParent() != CurLoop->getHeader())
     return false;
   auto DL = ExitBlock->getModule()->getDataLayout();
   auto *IVStart = LHS->getIncomingValueForBlock(CurLoop->getLoopPreheader());
-  auto *SimpleValOrNull = SimplifyICmpInst(ICI->getPredicate(),
+  auto *SimpleValOrNull = SimplifyCmpInst(Cond->getPredicate(),
                                           IVStart, RHS,
                                           {DL, /*TLI*/ nullptr,
                                               DT, /*AC*/ nullptr, BI});

llvm/test/Transforms/LICM/hoist-mustexec.ll

@@ -33,6 +33,34 @@ fail:
   ret i32 -1
 }
 
+; Same as test1, but with a floating point IR and fcmp
+define i32 @test_fcmp(i32* noalias nocapture readonly %a) nounwind uwtable {
+; CHECK-LABEL: @test_fcmp(
+entry:
+; CHECK: %i1 = load i32, i32* %a, align 4
+; CHECK-NEXT: br label %for.body
+  br label %for.body
+
+for.body:
+  %iv = phi float [ 0.0, %entry ], [ %inc, %continue ]
+  %acc = phi i32 [ 0, %entry ], [ %add, %continue ]
+  %r.chk = fcmp olt float %iv, 2000.0
+  br i1 %r.chk, label %continue, label %fail
+continue:
+  %i1 = load i32, i32* %a, align 4
+  %add = add nsw i32 %i1, %acc
+  %inc = fadd float %iv, 1.0
+  %exitcond = fcmp ogt float %inc, 1000.0
+  br i1 %exitcond, label %for.cond.cleanup, label %for.body
+
+for.cond.cleanup:
+  ret i32 %add
+
+fail:
+  call void @f()
+  ret i32 -1
+}
+
 ; Count down from a.length w/entry guard
 ; TODO: currently unable to prove the following:
 ; ule i32 (add nsw i32 %len, -1), %len where len is [0, 512]