[LV] Disable VPlan-based cost model for 19.x release.

[fhahn]

As discussed in #92555 flip the default for the option added in
#99536 to true.

This restores the original behavior for the release branch to give the VPlan-based cost model more time to mature on main.

[llvmbot]

@llvm/pr-subscribers-backend-risc-v

@llvm/pr-subscribers-llvm-transforms

Author: Florian Hahn (fhahn)

Changes

As discussed in #92555 flip the default for the option added in
#99536 to true.

This restores the original behavior for the release branch to give the VPlan-based cost model more time to mature on main.

---

Full diff: https://github.com/llvm/llvm-project/pull/100097.diff

2 Files Affected:

• (modified) llvm/lib/Transforms/Vectorize/LoopVectorize.cpp (+1-1)
• (modified) llvm/test/Transforms/LoopVectorize/RISCV/riscv-vector-reverse.ll (-2)

diff --git a/llvm/lib/Transforms/Vectorize/LoopVectorize.cpp b/llvm/lib/Transforms/Vectorize/LoopVectorize.cpp
index 6d28b8fabe42e..68363abdb817a 100644
--- a/llvm/lib/Transforms/Vectorize/LoopVectorize.cpp
+++ b/llvm/lib/Transforms/Vectorize/LoopVectorize.cpp
@@ -206,7 +206,7 @@ static cl::opt<unsigned> VectorizeMemoryCheckThreshold(
     cl::desc("The maximum allowed number of runtime memory checks"));
 
 static cl::opt<bool> UseLegacyCostModel(
-    "vectorize-use-legacy-cost-model", cl::init(false), cl::Hidden,
+    "vectorize-use-legacy-cost-model", cl::init(true), cl::Hidden,
     cl::desc("Use the legacy cost model instead of the VPlan-based cost model. "
              "This option will be removed in the future."));
 
diff --git a/llvm/test/Transforms/LoopVectorize/RISCV/riscv-vector-reverse.ll b/llvm/test/Transforms/LoopVectorize/RISCV/riscv-vector-reverse.ll
index fc310f4163082..1a78eaf644723 100644
--- a/llvm/test/Transforms/LoopVectorize/RISCV/riscv-vector-reverse.ll
+++ b/llvm/test/Transforms/LoopVectorize/RISCV/riscv-vector-reverse.ll
@@ -135,7 +135,6 @@ define void @vector_reverse_i64(ptr nocapture noundef writeonly %A, ptr nocaptur
 ; CHECK-NEXT:  LV: Interleaving is not beneficial.
 ; CHECK-NEXT:  LV: Found a vectorizable loop (vscale x 4) in <stdin>
 ; CHECK-NEXT:  LEV: Epilogue vectorization is not profitable for this loop
-; CHECK-NEXT:  VF picked by VPlan cost model: vscale x 4
 ; CHECK-NEXT:  Executing best plan with VF=vscale x 4, UF=1
 ; CHECK-NEXT:  VPlan 'Final VPlan for VF={vscale x 4},UF>=1' {
 ; CHECK-NEXT:  Live-in vp<%0> = VF * UF
@@ -339,7 +338,6 @@ define void @vector_reverse_f32(ptr nocapture noundef writeonly %A, ptr nocaptur
 ; CHECK-NEXT:  LV: Interleaving is not beneficial.
 ; CHECK-NEXT:  LV: Found a vectorizable loop (vscale x 4) in <stdin>
 ; CHECK-NEXT:  LEV: Epilogue vectorization is not profitable for this loop
-; CHECK-NEXT:  VF picked by VPlan cost model: vscale x 4
 ; CHECK-NEXT:  Executing best plan with VF=vscale x 4, UF=1
 ; CHECK-NEXT:  VPlan 'Final VPlan for VF={vscale x 4},UF>=1' {
 ; CHECK-NEXT:  Live-in vp<%0> = VF * UF

[nikic]

LGTM

[tru]

Merged manually as 183e8ec since I messed it up with a merge commit instead of a rebase. Sorry, learning the new flow.