[llvm] [SimplifyCFG] Don't perform "redirecting phis between unmergeable BBs and successor BB" optimization when not using jump tables. (PR #160560)

[via llvm-commits]

llvmbot wrote:


<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-llvm-transforms

Author: Amara Emerson (aemerson)

<details>
<summary>Changes</summary>

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

---
Full diff: https://github.com/llvm/llvm-project/pull/160560.diff


2 Files Affected:

- (modified) llvm/lib/Transforms/Utils/Local.cpp (+6) 
- (modified) llvm/test/Transforms/SimplifyCFG/branch-fold.ll (+34) 


``````````diff
diff --git a/llvm/lib/Transforms/Utils/Local.cpp b/llvm/lib/Transforms/Utils/Local.cpp
index 123881e276584..1a3d543160137 100644
--- a/llvm/lib/Transforms/Utils/Local.cpp
+++ b/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;
 
diff --git a/llvm/test/Transforms/SimplifyCFG/branch-fold.ll b/llvm/test/Transforms/SimplifyCFG/branch-fold.ll
index 8e7b91ea172be..2f4c5d9590e0e 100644
--- a/llvm/test/Transforms/SimplifyCFG/branch-fold.ll
+++ b/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}
 ;.

``````````

</details>


https://github.com/llvm/llvm-project/pull/160560