[SimplifyCFG] Don't perform "redirecting phis between unmergeable BBs and successor BB" optimization when not using jump tables.

This optimization works well for most cases but implicitly assumes that the
backend will be able to generate jump tables (see "And lookup table optimization
should convert it into add %arg 1." in PR #67275 description).

When -fno-jump-tables is used however this regresses code size, which is
especially painful in targets like armv7 where there's already high register
pressure.

rdar://157858055

Author: aemerson

llvm/lib/Transforms/Utils/Local.cpp

@@ -1159,6 +1159,12 @@ bool llvm::TryToSimplifyUncondBranchFromEmptyBlock(BasicBlock *BB,
   bool BBPhisMergeable = BBKillable || CanRedirectPredsOfEmptyBBToSucc(
                                            BB, Succ, BBPreds, CommonPred);
 
+  // If the function has the "no-jump-tables" attribute, merging phis
+  // can make size worse at -Oz.
+  auto *MF = BB->getParent();
+  if (MF->hasMinSize() && MF->hasFnAttribute("no-jump-tables"))
+    BBPhisMergeable = false;
+
   if ((!BBKillable && !BBPhisMergeable) || introduceTooManyPhiEntries(BB, Succ))
     return false;
 

llvm/test/Transforms/SimplifyCFG/branch-fold.ll

@@ -145,8 +145,42 @@ Succ:
   ret i8 %phi2
 }
 
+define i8 @common_pred_no_jt(i8 noundef %arg, i1 %c1, i1 %c2) #0 {
+; CHECK-LABEL: @common_pred_no_jt(
+; CHECK-NEXT:  Pred:
+; CHECK-NEXT:    call void @dummy()
+; CHECK-NEXT:    br i1 [[C1:%.*]], label [[COMMONPRED:%.*]], label [[SUCC:%.*]]
+; CHECK:       CommonPred:
+; CHECK-NEXT:    call void @dummy()
+; CHECK-NEXT:    br i1 [[C2:%.*]], label [[SUCC1:%.*]], label [[SUCC]]
+; CHECK:       BB:
+; CHECK-NEXT:    [[PHI1:%.*]] = phi i8 [ 0, [[PRED:%.*]] ], [ 1, [[COMMONPRED]] ]
+; CHECK-NEXT:    br label [[SUCC1]]
+; CHECK:       Succ:
+; CHECK-NEXT:    [[PHI2:%.*]] = phi i8 [ [[PHI1]], [[SUCC]] ], [ 4, [[COMMONPRED]] ]
+; CHECK-NEXT:    ret i8 [[PHI2]]
+;
+Pred:
+call void @dummy()
+  br i1 %c1, label %CommonPred, label %BB
+
+CommonPred:
+call void @dummy()
+  br i1 %c2, label %Succ, label %BB
+
+BB:
+  %phi1 = phi i8 [ 0, %Pred ], [1, %CommonPred]
+  br label %Succ
+
+Succ:
+  %phi2 = phi i8 [ %phi1, %BB ], [ 4, %CommonPred ]
+  ret i8 %phi2
+}
 declare void @dummy()
 
+attributes #0 = { minsize "no-jump-tables"="true" }
+
+
 !0 = !{!"branch_weights", i32 3, i32 7}
 !1 = !{!"branch_weights", i32 11, i32 4}
 ;.