[llvm] [SelectionDAG] Simplify vselect true, T, F -> T (PR #100992)

[via llvm-commits]

llvmbot wrote:


<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-llvm-selectiondag

Author: Luke Lau (lukel97)

<details>
<summary>Changes</summary>

This addresses a TODO where we can fold a vselect to it's true operand if the boolean is known to be all trues, checking getBooleanContents.


---
Full diff: https://github.com/llvm/llvm-project/pull/100992.diff


2 Files Affected:

- (modified) llvm/lib/CodeGen/SelectionDAG/SelectionDAG.cpp (+17-3) 
- (added) llvm/test/CodeGen/RISCV/rvv/vp-select.ll (+10) 


``````````diff
diff --git a/llvm/lib/CodeGen/SelectionDAG/SelectionDAG.cpp b/llvm/lib/CodeGen/SelectionDAG/SelectionDAG.cpp
index daebcdabda984..1876bf3f18c21 100644
--- a/llvm/lib/CodeGen/SelectionDAG/SelectionDAG.cpp
+++ b/llvm/lib/CodeGen/SelectionDAG/SelectionDAG.cpp
@@ -9927,13 +9927,27 @@ SDValue SelectionDAG::simplifySelect(SDValue Cond, SDValue T, SDValue F) {
   if (auto *CondC = dyn_cast<ConstantSDNode>(Cond))
     return CondC->isZero() ? F : T;
 
-  // TODO: This should simplify VSELECT with non-zero constant condition using
-  // something like this (but check boolean contents to be complete?):
   if (ConstantSDNode *CondC = isConstOrConstSplat(Cond, /*AllowUndefs*/ false,
-                                                  /*AllowTruncation*/ true))
+                                                  /*AllowTruncation*/ true)) {
     if (CondC->isZero())
       return F;
 
+    switch (TLI->getBooleanContents(Cond.getValueType())) {
+    case TargetLowering::UndefinedBooleanContent:
+      if (CondC->getAPIntValue()[0])
+        return T;
+      break;
+    case TargetLowering::ZeroOrOneBooleanContent:
+      if (CondC->isOne())
+        return T;
+      break;
+    case TargetLowering::ZeroOrNegativeOneBooleanContent:
+      if (CondC->isAllOnes())
+        return T;
+      break;
+    }
+  }
+
   // select ?, T, T --> T
   if (T == F)
     return T;
diff --git a/llvm/test/CodeGen/RISCV/rvv/vp-select.ll b/llvm/test/CodeGen/RISCV/rvv/vp-select.ll
new file mode 100644
index 0000000000000..5c903791927c6
--- /dev/null
+++ b/llvm/test/CodeGen/RISCV/rvv/vp-select.ll
@@ -0,0 +1,10 @@
+; NOTE: Assertions have been autogenerated by utils/update_llc_test_checks.py UTC_ARGS: --version 5
+; RUN: llc < %s -mtriple=riscv64 -mattr=+v -verify-machineinstrs | FileCheck %s
+
+define <vscale x 1 x i64> @all_ones(<vscale x 1 x i64> %true, <vscale x 1 x i64> %false, i32 %evl) {
+; CHECK-LABEL: all_ones:
+; CHECK:       # %bb.0:
+; CHECK-NEXT:    ret
+  %v = call <vscale x 1 x i64> @llvm.vp.select.nxv1i64(<vscale x 1 x i1> splat (i1 true), <vscale x 1 x i64> %true, <vscale x 1 x i64> %false, i32 %evl)
+  ret <vscale x 1 x i64> %v
+}

``````````

</details>


https://github.com/llvm/llvm-project/pull/100992