[llvm] [RISCV] Add FeaturePredictableSelectIsExpensive (PR #97708)

[Pengcheng Wang via llvm-commits]

https://github.com/wangpc-pp updated https://github.com/llvm/llvm-project/pull/97708

>From 1283fd013a372592bf79b1f6a6145a5ccaa4d39b Mon Sep 17 00:00:00 2001
From: Wang Pengcheng <wangpengcheng.pp at bytedance.com>
Date: Thu, 4 Jul 2024 18:24:48 +0800
Subject: [PATCH 1/2] =?UTF-8?q?[=F0=9D=98=80=F0=9D=97=BD=F0=9D=97=BF]=20in?=
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Created using spr 1.3.6-beta.1
---
 llvm/lib/Target/RISCV/RISCVFeatures.td        |  4 ++
 llvm/lib/Target/RISCV/RISCVISelLowering.cpp   |  5 ++
 ...ert-highly-predictable-select-to-branch.ll | 50 +++++++++++++++++++
 3 files changed, 59 insertions(+)
 create mode 100644 llvm/test/CodeGen/RISCV/convert-highly-predictable-select-to-branch.ll

diff --git a/llvm/lib/Target/RISCV/RISCVFeatures.td b/llvm/lib/Target/RISCV/RISCVFeatures.td
index 7b3057b187d5ed..5276e75c136c2a 100644
--- a/llvm/lib/Target/RISCV/RISCVFeatures.td
+++ b/llvm/lib/Target/RISCV/RISCVFeatures.td
@@ -1262,6 +1262,10 @@ def FeatureUnalignedVectorMem
 def FeaturePostRAScheduler : SubtargetFeature<"use-postra-scheduler",
     "UsePostRAScheduler", "true", "Schedule again after register allocation">;
 
+def FeaturePredictableSelectIsExpensive
+    : SubtargetFeature<"predictable-select-expensive", "IsPredictableSelectExpensive", "true",
+                       "Prefer likely predicted branches over selects">;
+
 def TuneNoOptimizedZeroStrideLoad
    : SubtargetFeature<"no-optimized-zero-stride-load", "HasOptimizedZeroStrideLoad",
                       "false", "Hasn't optimized (perform fewer memory operations)"
diff --git a/llvm/lib/Target/RISCV/RISCVISelLowering.cpp b/llvm/lib/Target/RISCV/RISCVISelLowering.cpp
index 45368a01a0a73e..05d91a283b41a9 100644
--- a/llvm/lib/Target/RISCV/RISCVISelLowering.cpp
+++ b/llvm/lib/Target/RISCV/RISCVISelLowering.cpp
@@ -1518,6 +1518,11 @@ RISCVTargetLowering::RISCVTargetLowering(const TargetMachine &TM,
 
   // Disable strict node mutation.
   IsStrictFPEnabled = true;
+
+  // Let the subtarget decide if a predictable select is more expensive than the
+  // corresponding branch. This information is used in CGP/SelectOpt to decide
+  // when to convert selects into branches.
+  PredictableSelectIsExpensive = Subtarget.isPredictableSelectExpensive();
 }
 
 EVT RISCVTargetLowering::getSetCCResultType(const DataLayout &DL,
diff --git a/llvm/test/CodeGen/RISCV/convert-highly-predictable-select-to-branch.ll b/llvm/test/CodeGen/RISCV/convert-highly-predictable-select-to-branch.ll
new file mode 100644
index 00000000000000..1abd774d7a4874
--- /dev/null
+++ b/llvm/test/CodeGen/RISCV/convert-highly-predictable-select-to-branch.ll
@@ -0,0 +1,50 @@
+; NOTE: Assertions have been autogenerated by utils/update_llc_test_checks.py UTC_ARGS: --version 5
+; RUN: llc -mtriple=riscv64 -mattr=+zicond < %s | FileCheck %s --check-prefixes=CHECK,CHEAP
+; RUN: llc -mtriple=riscv64 -mattr=+zicond,+predictable-select-expensive < %s | FileCheck %s --check-prefixes=CHECK,EXPENSIVE
+
+; Test has not predictable select, which should not be transformed to a branch
+define i32 @test1(i32 %a) {
+; CHECK-LABEL: test1:
+; CHECK:       # %bb.0: # %entry
+; CHECK-NEXT:    sext.w a1, a0
+; CHECK-NEXT:    slti a1, a1, 1
+; CHECK-NEXT:    addiw a0, a0, -1
+; CHECK-NEXT:    czero.nez a0, a0, a1
+; CHECK-NEXT:    ret
+entry:
+  %cmp = icmp slt i32 %a, 1
+  %dec = sub i32 %a, 1
+  %res = select i1 %cmp, i32 0, i32 %dec, !prof !0
+  ret i32 %res
+}
+
+; Test has highly predictable select according to profile data,
+; which should be transformed to a branch on cores with enabled FeaturePredictableSelectIsExpensive
+define i32 @test2(i32 %a) {
+; CHEAP-LABEL: test2:
+; CHEAP:       # %bb.0: # %entry
+; CHEAP-NEXT:    sext.w a1, a0
+; CHEAP-NEXT:    slti a1, a1, 1
+; CHEAP-NEXT:    addiw a0, a0, -1
+; CHEAP-NEXT:    czero.nez a0, a0, a1
+; CHEAP-NEXT:    ret
+;
+; EXPENSIVE-LABEL: test2:
+; EXPENSIVE:       # %bb.0: # %entry
+; EXPENSIVE-NEXT:    sext.w a1, a0
+; EXPENSIVE-NEXT:    blez a1, .LBB1_2
+; EXPENSIVE-NEXT:  # %bb.1: # %select.false
+; EXPENSIVE-NEXT:    addiw a0, a0, -1
+; EXPENSIVE-NEXT:    ret
+; EXPENSIVE-NEXT:  .LBB1_2:
+; EXPENSIVE-NEXT:    li a0, 0
+; EXPENSIVE-NEXT:    ret
+entry:
+  %cmp = icmp slt i32 %a, 1
+  %dec = sub i32 %a, 1
+  %res = select i1 %cmp, i32 0, i32 %dec, !prof !1
+  ret i32 %res
+}
+
+!0 = !{!"branch_weights", i32 1, i32 1}
+!1 = !{!"branch_weights", i32 1, i32 1000}

>From 64312ac1f69b7285ac030da3e285a148d9917a8b Mon Sep 17 00:00:00 2001
From: Wang Pengcheng <wangpengcheng.pp at bytedance.com>
Date: Fri, 5 Jul 2024 10:57:49 +0800
Subject: [PATCH 2/2] Rename flag to PredictableSelectIsExpensive

Created using spr 1.3.6-beta.1
---
 llvm/lib/Target/RISCV/RISCVFeatures.td      | 2 +-
 llvm/lib/Target/RISCV/RISCVISelLowering.cpp | 2 +-
 2 files changed, 2 insertions(+), 2 deletions(-)

diff --git a/llvm/lib/Target/RISCV/RISCVFeatures.td b/llvm/lib/Target/RISCV/RISCVFeatures.td
index 5276e75c136c2a..697a22b30dfcca 100644
--- a/llvm/lib/Target/RISCV/RISCVFeatures.td
+++ b/llvm/lib/Target/RISCV/RISCVFeatures.td
@@ -1263,7 +1263,7 @@ def FeaturePostRAScheduler : SubtargetFeature<"use-postra-scheduler",
     "UsePostRAScheduler", "true", "Schedule again after register allocation">;
 
 def FeaturePredictableSelectIsExpensive
-    : SubtargetFeature<"predictable-select-expensive", "IsPredictableSelectExpensive", "true",
+    : SubtargetFeature<"predictable-select-expensive", "PredictableSelectIsExpensive", "true",
                        "Prefer likely predicted branches over selects">;
 
 def TuneNoOptimizedZeroStrideLoad
diff --git a/llvm/lib/Target/RISCV/RISCVISelLowering.cpp b/llvm/lib/Target/RISCV/RISCVISelLowering.cpp
index 05d91a283b41a9..bef8cf59f07b7b 100644
--- a/llvm/lib/Target/RISCV/RISCVISelLowering.cpp
+++ b/llvm/lib/Target/RISCV/RISCVISelLowering.cpp
@@ -1522,7 +1522,7 @@ RISCVTargetLowering::RISCVTargetLowering(const TargetMachine &TM,
   // Let the subtarget decide if a predictable select is more expensive than the
   // corresponding branch. This information is used in CGP/SelectOpt to decide
   // when to convert selects into branches.
-  PredictableSelectIsExpensive = Subtarget.isPredictableSelectExpensive();
+  PredictableSelectIsExpensive = Subtarget.predictableSelectIsExpensive();
 }
 
 EVT RISCVTargetLowering::getSetCCResultType(const DataLayout &DL,