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

[via llvm-commits]

llvmbot wrote:


<!--LLVM PR SUMMARY COMMENT-->

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

Author: Pengcheng Wang (wangpc-pp)

<details>
<summary>Changes</summary>

This information is used in CGP/SelectOpt to decide when to convert
selects into branches.


---
Full diff: https://github.com/llvm/llvm-project/pull/97708.diff


3 Files Affected:

- (modified) llvm/lib/Target/RISCV/RISCVFeatures.td (+4) 
- (modified) llvm/lib/Target/RISCV/RISCVISelLowering.cpp (+5) 
- (added) llvm/test/CodeGen/RISCV/convert-highly-predictable-select-to-branch.ll (+50) 


``````````diff
diff --git a/llvm/lib/Target/RISCV/RISCVFeatures.td b/llvm/lib/Target/RISCV/RISCVFeatures.td
index 7b3057b187d5e..5276e75c136c2 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 45368a01a0a73..05d91a283b41a 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 0000000000000..1abd774d7a487
--- /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}

``````````

</details>


https://github.com/llvm/llvm-project/pull/97708