[llvm] [InstCombine] Eliminate icmp+zext pairs over phis more aggressively (PR #121767)

[Nikita Popov via llvm-commits]

https://github.com/nikic created https://github.com/llvm/llvm-project/pull/121767

None

>From 05b4f76f6e57d25886859947238a975b62790e1a Mon Sep 17 00:00:00 2001
From: Nikita Popov <npopov at redhat.com>
Date: Mon, 6 Jan 2025 15:03:33 +0100
Subject: [PATCH] [InstCombine] Eliminate icmp+zext pairs over phis more
 aggressively

---
 .../InstCombine/InstructionCombining.cpp      | 29 +++++++++++++++----
 1 file changed, 23 insertions(+), 6 deletions(-)

diff --git a/llvm/lib/Transforms/InstCombine/InstructionCombining.cpp b/llvm/lib/Transforms/InstCombine/InstructionCombining.cpp
index f63de1f0d410e2..553435c937a70a 100644
--- a/llvm/lib/Transforms/InstCombine/InstructionCombining.cpp
+++ b/llvm/lib/Transforms/InstCombine/InstructionCombining.cpp
@@ -1822,12 +1822,29 @@ Instruction *InstCombinerImpl::foldOpIntoPhi(Instruction &I, PHINode *PN,
       continue;
     }
 
-    // If the only use of phi is comparing it with a constant then we can
-    // put this comparison in the incoming BB directly after a ucmp/scmp call
-    // because we know that it will simplify to a single icmp.
-    const APInt *Ignored;
-    if (isa<CmpIntrinsic>(InVal) && InVal->hasOneUser() &&
-        match(&I, m_ICmp(m_Specific(PN), m_APInt(Ignored)))) {
+    // Handle some cases that can't be fully simplified, but where we know that
+    // the two instructions will fold into one.
+    auto WillFold = [&]() {
+      if (!InVal->hasOneUser())
+        return false;
+
+      // icmp of ucmp/scmp with constant will fold to icmp.
+      const APInt *Ignored;
+      if (isa<CmpIntrinsic>(InVal) &&
+          match(&I, m_ICmp(m_Specific(PN), m_APInt(Ignored))))
+        return true;
+
+      // icmp eq zext(bool), 0 will fold to !bool.
+      if (isa<ZExtInst>(InVal) &&
+          cast<ZExtInst>(InVal)->getSrcTy()->isIntOrIntVectorTy(1) &&
+          match(&I,
+                m_SpecificICmp(ICmpInst::ICMP_EQ, m_Specific(PN), m_Zero())))
+        return true;
+
+      return false;
+    };
+
+    if (WillFold()) {
       OpsToMoveUseToIncomingBB.push_back(i);
       NewPhiValues.push_back(nullptr);
       continue;