[llvm] [ValueTracking] Compute KnownFP state from recursive select/phi. (PR #113686)

[David Green via llvm-commits]

https://github.com/davemgreen created https://github.com/llvm/llvm-project/pull/113686

Given a recursive phi with select:
 %p = phi [ 0, entry ], [ %sel, loop]
 %sel = select %c, %other, %p

The fp state can be calculated using the knowledge that the select/phi pair can only be the initial state (0 here) or from %other. This adds a short-cut into computeKnownFPClass for PHI to detect that the select is recursive back to the phi, and if so use the state from the other operand.

This helps to address a regression from #83200.

>From 411c89ebd1d9067ab529a0b82f3a1e8c4caf2b11 Mon Sep 17 00:00:00 2001
From: David Green <david.green at arm.com>
Date: Fri, 25 Oct 2024 14:39:23 +0100
Subject: [PATCH] [ValueTracking] Compute KnownFP state from recursive
 select/phi.

Given a recursive phi with select:
 %p = phi [ 0, entry ], [ %sel, loop]
 %sel = select %c, %other, %p

The fp state can be calculated using the knowledge that the select/phi pair can
only be the initial state (0 here) or from %other. This adds a short-cut into
computeKnownFPClass for PHI to detect that the select is recursive back to the
phi, and if so use the state from the other operand.

This helps to address a regression from #83200.
---
 llvm/lib/Analysis/ValueTracking.cpp                        | 7 +++++++
 .../Transforms/LoopVectorize/ARM/mve-selectandorcost.ll    | 4 ++--
 2 files changed, 9 insertions(+), 2 deletions(-)

diff --git a/llvm/lib/Analysis/ValueTracking.cpp b/llvm/lib/Analysis/ValueTracking.cpp
index e9ed8b3c862b55..708922e21ee0bb 100644
--- a/llvm/lib/Analysis/ValueTracking.cpp
+++ b/llvm/lib/Analysis/ValueTracking.cpp
@@ -5999,6 +5999,13 @@ void computeKnownFPClass(const Value *V, const APInt &DemandedElts,
         if (IncValue == P)
           continue;
 
+        // If the Use is a select of this phi, use the fp class of the other
+        // operand to break the recursion.
+        Value *V;
+        if (match(IncValue, m_Select(m_Value(), m_Specific(P), m_Value(V))) ||
+            match(IncValue, m_Select(m_Value(), m_Value(V), m_Specific(P))))
+          IncValue = V;
+
         KnownFPClass KnownSrc;
         // Recurse, but cap the recursion to two levels, because we don't want
         // to waste time spinning around in loops. We need at least depth 2 to
diff --git a/llvm/test/Transforms/LoopVectorize/ARM/mve-selectandorcost.ll b/llvm/test/Transforms/LoopVectorize/ARM/mve-selectandorcost.ll
index fc56754166d609..b54d4a97d07be7 100644
--- a/llvm/test/Transforms/LoopVectorize/ARM/mve-selectandorcost.ll
+++ b/llvm/test/Transforms/LoopVectorize/ARM/mve-selectandorcost.ll
@@ -45,8 +45,8 @@ define float @test(ptr nocapture readonly %pA, ptr nocapture readonly %pB, i32 %
 ; CHECK-NEXT:    [[TMP7:%.*]] = fsub fast <4 x float> [[WIDE_LOAD]], [[WIDE_LOAD7]]
 ; CHECK-NEXT:    [[TMP8:%.*]] = call fast <4 x float> @llvm.fabs.v4f32(<4 x float> [[TMP7]])
 ; CHECK-NEXT:    [[TMP9:%.*]] = fdiv fast <4 x float> [[TMP8]], [[TMP6]]
-; CHECK-NEXT:    [[TMP10:%.*]] = fadd fast <4 x float> [[TMP9]], [[VEC_PHI]]
-; CHECK-NEXT:    [[PREDPHI]] = select <4 x i1> [[DOTNOT9]], <4 x float> [[VEC_PHI]], <4 x float> [[TMP10]]
+; CHECK-NEXT:    [[TMP10:%.*]] = select <4 x i1> [[DOTNOT9]], <4 x float> <float -0.000000e+00, float -0.000000e+00, float -0.000000e+00, float -0.000000e+00>, <4 x float> [[TMP9]]
+; CHECK-NEXT:    [[PREDPHI]] = fadd fast <4 x float> [[VEC_PHI]], [[TMP10]]
 ; CHECK-NEXT:    [[INDEX_NEXT]] = add nuw i32 [[INDEX]], 4
 ; CHECK-NEXT:    [[TMP11:%.*]] = icmp eq i32 [[INDEX_NEXT]], [[N_VEC]]
 ; CHECK-NEXT:    br i1 [[TMP11]], label [[MIDDLE_BLOCK:%.*]], label [[VECTOR_BODY]], !llvm.loop [[LOOP0:![0-9]+]]