[llvm] [ValueTracking] Don't special case depth for phi of select (PR #114996)

[Nikita Popov via llvm-commits]

https://github.com/nikic created https://github.com/llvm/llvm-project/pull/114996

As discussed on https://github.com/llvm/llvm-project/pull/114689#pullrequestreview-2411822612 and following, there is no principled reason why the phi of select case should have a different recursion limit than the general case. There may still be fan-out, and there may still be indirect recursion. Revert that part of #113707.

>From 96f345d2e09bcff5ed0ba50e36bcd5806289fed1 Mon Sep 17 00:00:00 2001
From: Nikita Popov <npopov at redhat.com>
Date: Tue, 5 Nov 2024 15:18:58 +0100
Subject: [PATCH] [ValueTracking] Don't special case depth for phi of select

As discussed on https://github.com/llvm/llvm-project/pull/114689#pullrequestreview-2411822612 and following, there is no principled reason why
the phi of select case should have a different recursion limit
than the general case. There may still be fan-out, and there may
still be indirect recursion. Revert that part of #113707.
---
 llvm/lib/Analysis/ValueTracking.cpp | 18 ++++++++----------
 1 file changed, 8 insertions(+), 10 deletions(-)

diff --git a/llvm/lib/Analysis/ValueTracking.cpp b/llvm/lib/Analysis/ValueTracking.cpp
index 5c20c24d0ae00a..7d8e0d65dbfebb 100644
--- a/llvm/lib/Analysis/ValueTracking.cpp
+++ b/llvm/lib/Analysis/ValueTracking.cpp
@@ -1566,20 +1566,12 @@ static void computeKnownBitsFromOperator(const Operator *I,
         // Skip direct self references.
         if (IncValue == P) continue;
 
-        // Recurse, but cap the recursion to one level, because we don't
-        // want to waste time spinning around in loops.
-        // TODO: See if we can base recursion limiter on number of incoming phi
-        // edges so we don't overly clamp analysis.
-        unsigned IncDepth = MaxAnalysisRecursionDepth - 1;
-
         // If the Use is a select of this phi, use the knownbit of the other
         // operand to break the recursion.
         if (auto *SI = dyn_cast<SelectInst>(IncValue)) {
-          if (SI->getTrueValue() == P || SI->getFalseValue() == P) {
+          if (SI->getTrueValue() == P || SI->getFalseValue() == P)
             IncValue = SI->getTrueValue() == P ? SI->getFalseValue()
                                                : SI->getTrueValue();
-            IncDepth = Depth + 1;
-          }
         }
 
         // Change the context instruction to the "edge" that flows into the
@@ -1590,7 +1582,13 @@ static void computeKnownBitsFromOperator(const Operator *I,
         RecQ.CxtI = P->getIncomingBlock(u)->getTerminator();
 
         Known2 = KnownBits(BitWidth);
-        computeKnownBits(IncValue, DemandedElts, Known2, IncDepth, RecQ);
+
+        // Recurse, but cap the recursion to one level, because we don't
+        // want to waste time spinning around in loops.
+        // TODO: See if we can base recursion limiter on number of incoming phi
+        // edges so we don't overly clamp analysis.
+        computeKnownBits(IncValue, DemandedElts, Known2,
+                         MaxAnalysisRecursionDepth - 1, RecQ);
 
         // See if we can further use a conditional branch into the phi
         // to help us determine the range of the value.