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

[reames]

Summary

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 IV 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).

[anna]

lgtm!

llvm/trunk/lib/Transforms/Utils/LoopUtils.cpp

static bool CanProveNotTakenFirstIteration(BasicBlock *ExitBlock,
   auto *CondExitBlock = ExitBlock->getSinglePredecessor();
   if (!CondExitBlock)
     // expect unique exits
     return false;
   assert(CurLoop->contains(CondExitBlock) && "meaning of exit block");
   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(),
-                                           IVStart, RHS,
-                                           {DL, /*TLI*/ nullptr,
-                                               DT, /*AC*/ nullptr, BI});
+  auto *SimpleValOrNull = SimplifyCmpInst(Cond->getPredicate(),
   auto *SimpleCst = dyn_cast_or_null<Constant>(SimpleValOrNull);
   if (!SimpleCst)
     return false;
   if (ExitBlock == BI->getSuccessor(0))
     return SimpleCst->isZeroValue();
   assert(ExitBlock == BI->getSuccessor(1) && "implied by above");
   return SimpleCst->isAllOnesValue();
 }

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

 for.cond.cleanup:
   ret i32 %add
 
 fail:
   call void @f()
   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]
 define i32 @test2(i32* noalias nocapture readonly %a) nounwind uwtable {
 ; CHECK-LABEL: @test2(
 entry:
   %len = load i32, i32* %a, align 4, !range !{i32 0, i32 512}
   %is.non.pos = icmp eq i32 %len, 0