ARM64-SVE: Fix conditional select for Zeroing predicates

Author: a74nh

src/coreclr/jit/hwintrinsic.h

@@ -236,16 +236,18 @@ enum HWIntrinsicFlag : unsigned int
     // then the intrinsic should be switched to a scalar only version.
     HW_Flag_HasScalarInputVariant = 0x2000000,
 
+    // The intrinsic uses a mask in arg1 to select elements present in the result, and must use a low vector register.
+    HW_Flag_LowVectorOperation = 0x4000000,
+
+    // The intrinsic uses a mask in arg1 to select elements present in the result, which zeros inactive elements
+    // (instead of merging).
+    HW_Flag_ZeroingMaskedOperation = 0x8000000,
+
 #endif // TARGET_XARCH
 
     // The intrinsic is a FusedMultiplyAdd intrinsic
     HW_Flag_FmaIntrinsic = 0x40000000,
 
-#if defined(TARGET_ARM64)
-    // The intrinsic uses a mask in arg1 to select elements present in the result, and must use a low vector register.
-    HW_Flag_LowVectorOperation = 0x4000000,
-#endif
-
     HW_Flag_CanBenefitFromConstantProp = 0x80000000,
 };
 
@@ -955,6 +957,12 @@ struct HWIntrinsicInfo
         return (flags & HW_Flag_HasScalarInputVariant) != 0;
     }
 
+    static bool IsZeroingMaskedOperation(NamedIntrinsic id)
+    {
+        const HWIntrinsicFlag flags = lookupFlags(id);
+        return (flags & HW_Flag_ZeroingMaskedOperation) != 0;
+    }
+
     static NamedIntrinsic GetScalarInputVariant(NamedIntrinsic id)
     {
         assert(HasScalarInputVariant(id));

src/coreclr/jit/hwintrinsiclistarm64sve.h

@@ -4028,9 +4028,13 @@ GenTree* Lowering::LowerHWIntrinsicCndSel(GenTreeHWIntrinsic* cndSelNode)
         // `trueValue`
         GenTreeHWIntrinsic* nestedCndSel = op2->AsHWIntrinsic();
         GenTree*            nestedOp1    = nestedCndSel->Op(1);
+        GenTree*            nestedOp2    = nestedCndSel->Op(2);
         assert(varTypeIsMask(nestedOp1));
+        assert(nestedOp2->OperIsHWIntrinsic());
 
-        if (nestedOp1->IsMaskAllBitsSet())
+        NamedIntrinsic nestedOp2Id = nestedOp2->AsHWIntrinsic()->GetHWIntrinsicId();
+
+        if (nestedOp1->IsMaskAllBitsSet() && !HWIntrinsicInfo::IsZeroingMaskedOperation(nestedOp2Id))
         {
             GenTree* nestedOp2 = nestedCndSel->Op(2);
             GenTree* nestedOp3 = nestedCndSel->Op(3);

src/coreclr/jit/lowerarmarch.cpp

@@ -56,17 +56,7 @@ namespace JIT.HardwareIntrinsics.Arm
                 test.RunStructFldScenario();
 
                 // Validates using inside ConditionalSelect with value falseValue
-                // Currently, using this operation in ConditionalSelect() gives incorrect result
-                // when falseReg == targetReg because this instruction uses Pg/Z to update the targetReg
-                // instead of Pg/M to merge it. As such, the value of falseReg is lost. Ideally, such
-                // instructions should be marked similar to RMW (a different flag name) to make sure that
-                // we do not assign falseReg/targetReg same. Then, we would do something like this:
-                //
-                // ldnf1sh target, pg/z, [x0]
-                // sel mask, target, target, falseReg
-                //
-                // This needs more careful thinking, so disabling it for now.
-                // test.ConditionalSelect_FalseOp();
+                test.ConditionalSelect_FalseOp();
 
                 // Validates using inside ConditionalSelect with zero falseValue
                 test.ConditionalSelect_ZeroOp();

src/tests/JIT/HardwareIntrinsics/Arm/Shared/SveGatherVectorByteOffsets.template

@@ -56,17 +56,7 @@ namespace JIT.HardwareIntrinsics.Arm
                 test.RunStructFldScenario();
 
                 // Validates using inside ConditionalSelect with value falseValue
-                // Currently, using this operation in ConditionalSelect() gives incorrect result
-                // when falseReg == targetReg because this instruction uses Pg/Z to update the targetReg
-                // instead of Pg/M to merge it. As such, the value of falseReg is lost. Ideally, such
-                // instructions should be marked similar to RMW (a different flag name) to make sure that
-                // we do not assign falseReg/targetReg same. Then, we would do something like this:
-                //
-                // ldnf1sh target, pg/z, [x0]
-                // sel mask, target, target, falseReg
-                //
-                // This needs more careful thinking, so disabling it for now.
-                // test.ConditionalSelect_FalseOp();
+                test.ConditionalSelect_FalseOp();
 
                 // Validates using inside ConditionalSelect with zero falseValue
                 test.ConditionalSelect_ZeroOp();

src/tests/JIT/HardwareIntrinsics/Arm/Shared/SveGatherVectorIndices.template

@@ -56,17 +56,7 @@ namespace JIT.HardwareIntrinsics.Arm
                 test.RunStructFldScenario();
 
                 // Validates using inside ConditionalSelect with value falseValue
-                // Currently, using this operation in ConditionalSelect() gives incorrect result
-                // when falseReg == targetReg because this instruction uses Pg/Z to update the targetReg
-                // instead of Pg/M to merge it. As such, the value of falseReg is lost. Ideally, such
-                // instructions should be marked similar to RMW (a different flag name) to make sure that
-                // we do not assign falseReg/targetReg same. Then, we would do something like this:
-                //
-                // ldnf1sh target, pg/z, [x0]
-                // sel mask, target, target, falseReg
-                //
-                // This needs more careful thinking, so disabling it for now.
-                // test.ConditionalSelect_FalseOp();
+                test.ConditionalSelect_FalseOp();
 
                 // Validates using inside ConditionalSelect with zero falseValue
                 test.ConditionalSelect_ZeroOp();

src/tests/JIT/HardwareIntrinsics/Arm/Shared/SveGatherVectorVectorBases.template

@@ -47,17 +47,7 @@ namespace JIT.HardwareIntrinsics.Arm
                 test.RunStructFldScenario();
 
                 // Validates using inside ConditionalSelect with value falseValue
-                // Currently, using this operation in ConditionalSelect() gives incorrect result
-                // when falseReg == targetReg because this instruction uses Pg/Z to update the targetReg
-                // instead of Pg/M to merge it. As such, the value of falseReg is lost. Ideally, such
-                // instructions should be marked similar to RMW (a different flag name) to make sure that
-                // we do not assign falseReg/targetReg same. Then, we would do something like this:
-                //
-                // ldnf1sh target, pg/z, [x0]
-                // sel mask, target, target, falseReg
-                //
-                // This needs more careful thinking, so disabling it for now.
-                // test.ConditionalSelect_FalseOp();
+                test.ConditionalSelect_FalseOp();
 
                 // Validates using inside ConditionalSelect with zero falseValue
                 test.ConditionalSelect_ZeroOp();