[llvm] [KnownFPClass] Unify and refine interested classes for `KnownFPClass::log` (PR #219751)

[Eric Ross via llvm-commits]

https://github.com/ZERICO2005 updated https://github.com/llvm/llvm-project/pull/219751

>From 70c817c6eeeab4bc38af884ff0100dff8b0456f6 Mon Sep 17 00:00:00 2001
From: zerico <zerico2005 at gmail.com>
Date: Sat, 29 Aug 2026 20:44:08 -0600
Subject: [PATCH] [KnownFPClass] Unify and refine interested classes for
 KnownFPClass::log

---
 llvm/lib/Analysis/ValueTracking.cpp           | 47 +++++++++++--------
 .../CodeGen/GlobalISel/GISelValueTracking.cpp | 31 ++++++++----
 llvm/unittests/Analysis/ValueTrackingTest.cpp | 12 +++++
 .../CodeGen/GlobalISel/KnownFPClassTest.cpp   | 24 ++++++++++
 4 files changed, 85 insertions(+), 29 deletions(-)

diff --git a/llvm/lib/Analysis/ValueTracking.cpp b/llvm/lib/Analysis/ValueTracking.cpp
index 1580571a01451..0ba071eab51f8 100644
--- a/llvm/lib/Analysis/ValueTracking.cpp
+++ b/llvm/lib/Analysis/ValueTracking.cpp
@@ -5627,29 +5627,38 @@ void computeKnownFPClass(const Value *V, const APInt &DemandedElts,
     case Intrinsic::experimental_constrained_log10:
     case Intrinsic::experimental_constrained_log2:
     case Intrinsic::amdgcn_log: {
-      Type *EltTy = II->getType()->getScalarType();
+      FPClassTest InterestedSrcs = fcNone;
 
-      // log(+inf) -> +inf
-      // log([+-]0.0) -> -inf
-      // log(-inf) -> nan
-      // log(-x) -> nan
-      if ((InterestedClasses & (fcNan | fcInf)) != fcNone) {
-        FPClassTest InterestedSrcs = InterestedClasses;
-        if ((InterestedClasses & fcNegInf) != fcNone)
-          InterestedSrcs |= fcZero | fcSubnormal;
-        if ((InterestedClasses & fcNan) != fcNone)
-          InterestedSrcs |= fcNan | fcNegative;
+      // log(negative) produces NaN.
+      if ((InterestedClasses & fcNan) != fcNone)
+        InterestedSrcs |= fcNan | fcNegative;
 
-        KnownFPClass KnownSrc;
-        computeKnownFPClass(II->getArgOperand(0), DemandedElts, InterestedSrcs,
-                            KnownSrc, Q, Depth + 1);
+      // log(logical-zero) produces negative infinity.
+      if ((InterestedClasses & fcNegInf) != fcNone)
+        InterestedSrcs |= fcZero | fcSubnormal;
 
-        const Function *F = II->getFunction();
-        DenormalMode Mode = F ? F->getDenormalMode(EltTy->getFltSemantics())
-                              : DenormalMode::getDynamic();
-        Known = KnownFPClass::log(KnownSrc, Mode);
-      }
+      // log(x) < -0.0 if x < +1.0
+      if ((InterestedClasses & fcNegNormal) != fcNone)
+        InterestedSrcs |= fcPosSubnormal | fcPosNormal;
+
+      // log(x) >= +0.0 if x >= +1.0
+      if ((InterestedClasses & (fcPosZero | fcPosNormal)) != fcNone)
+        InterestedSrcs |= fcPosNormal;
+
+      // log(x) is positive infinity iff x is positive infinity.
+      if ((InterestedClasses & fcPosInf) != fcNone)
+        InterestedSrcs |= fcPosInf;
 
+      KnownFPClass KnownSrc;
+      if (InterestedSrcs != fcNone)
+        computeKnownFPClass(II->getArgOperand(0), DemandedElts, InterestedSrcs,
+                            KnownSrc, Q, Depth + 1);
+      const Function *F = II->getFunction();
+      DenormalMode Mode =
+          F ? F->getDenormalMode(
+                  II->getType()->getScalarType()->getFltSemantics())
+            : DenormalMode::getDynamic();
+      Known = KnownFPClass::log(KnownSrc, Mode);
       break;
     }
     case Intrinsic::pow: {
diff --git a/llvm/lib/CodeGen/GlobalISel/GISelValueTracking.cpp b/llvm/lib/CodeGen/GlobalISel/GISelValueTracking.cpp
index 0bbafeb6dca41..4cdbeeebc39bb 100644
--- a/llvm/lib/CodeGen/GlobalISel/GISelValueTracking.cpp
+++ b/llvm/lib/CodeGen/GlobalISel/GISelValueTracking.cpp
@@ -1612,22 +1612,33 @@ void GISelValueTracking::computeKnownFPClass(Register R,
   case TargetOpcode::G_FLOG:
   case TargetOpcode::G_FLOG2:
   case TargetOpcode::G_FLOG10: {
-    // log(+inf) -> +inf
-    // log([+-]0.0) -> -inf
-    // log(-inf) -> nan
-    // log(-x) -> nan
-    if ((InterestedClasses & (fcNan | fcInf)) == fcNone)
-      break;
+    FPClassTest InterestedSrcs = fcNone;
 
-    FPClassTest InterestedSrcs = InterestedClasses;
-    if ((InterestedClasses & fcNegInf) != fcNone)
-      InterestedSrcs |= fcZero | fcSubnormal;
+    // log(negative) produces NaN.
     if ((InterestedClasses & fcNan) != fcNone)
       InterestedSrcs |= fcNan | fcNegative;
 
+    // log(logical-zero) produces negative infinity.
+    if ((InterestedClasses & fcNegInf) != fcNone)
+      InterestedSrcs |= fcZero | fcSubnormal;
+
+    // log(x) < -0.0 if x < +1.0
+    if ((InterestedClasses & fcNegNormal) != fcNone)
+      InterestedSrcs |= fcPosSubnormal | fcPosNormal;
+
+    // log(x) >= +0.0 if x >= +1.0
+    if ((InterestedClasses & (fcPosZero | fcPosNormal)) != fcNone)
+      InterestedSrcs |= fcPosNormal;
+
+    // log(x) is positive infinity iff x is positive infinity.
+    if ((InterestedClasses & fcPosInf) != fcNone)
+      InterestedSrcs |= fcPosInf;
+
     Register Val = MI.getOperand(1).getReg();
     KnownFPClass KnownSrc;
-    computeKnownFPClass(Val, DemandedElts, InterestedSrcs, KnownSrc, Depth + 1);
+    if (InterestedSrcs != fcNone)
+      computeKnownFPClass(Val, DemandedElts, InterestedSrcs, KnownSrc,
+                          Depth + 1);
 
     LLT Ty = MRI.getType(Val).getScalarType();
     const fltSemantics &FltSem = getFltSemanticForLLT(Ty);
diff --git a/llvm/unittests/Analysis/ValueTrackingTest.cpp b/llvm/unittests/Analysis/ValueTrackingTest.cpp
index 0634f43f3e79c..9635c48525591 100644
--- a/llvm/unittests/Analysis/ValueTrackingTest.cpp
+++ b/llvm/unittests/Analysis/ValueTrackingTest.cpp
@@ -1756,6 +1756,18 @@ TEST_F(ComputeKnownFPClassTest, CopySignNInfSrc0_PosSign) {
   expectKnownFPClass(fcPosZero | fcPosNormal | fcNan, false);
 }
 
+TEST_F(ComputeKnownFPClassTest, LogDeduceSubnormalOrNegativeZero) {
+  parseAssembly("declare float @llvm.log.f32(float)\n"
+                "define float @test(float %x) {\n"
+                "  %A = call float @llvm.log.f32(float %x)\n"
+                "  ret float %A\n"
+                "}\n");
+
+  KnownFPClass Known =
+      computeKnownFPClass(A, M->getDataLayout(), fcNegZero | fcSubnormal);
+  EXPECT_EQ(~(fcNegZero | fcSubnormal), Known.getKnownFPClasses());
+}
+
 TEST_F(ComputeKnownFPClassTest, UIToFP) {
   parseAssembly(
       "define float @test(i32 %arg0, i16 %arg1) {\n"
diff --git a/llvm/unittests/CodeGen/GlobalISel/KnownFPClassTest.cpp b/llvm/unittests/CodeGen/GlobalISel/KnownFPClassTest.cpp
index d1b817934a0fd..ffdf362fec720 100644
--- a/llvm/unittests/CodeGen/GlobalISel/KnownFPClassTest.cpp
+++ b/llvm/unittests/CodeGen/GlobalISel/KnownFPClassTest.cpp
@@ -878,6 +878,30 @@ TEST_F(AArch64GISelMITest, TestFPClassFLogNeg) {
   EXPECT_EQ(std::nullopt, Known.getSignBit());
 }
 
+TEST_F(AArch64GISelMITest, TestFPClassFLogDeduceSubnormalOrNegativeZero) {
+  StringRef MIRString = R"(
+    %ptr:_(p0) = G_IMPLICIT_DEF
+    %val:_(s32) = G_LOAD %ptr(p0) :: (load (s32))
+    %flog:_(s32) = G_FLOG %val
+    %copy_flog:_(s32) = COPY %flog
+)";
+
+  setUp(MIRString);
+  if (!TM)
+    GTEST_SKIP();
+
+  Register CopyReg = Copies[Copies.size() - 1];
+  MachineInstr *FinalCopy = MRI->getVRegDef(CopyReg);
+  Register SrcReg = FinalCopy->getOperand(1).getReg();
+
+  GISelValueTracking Info(*MF);
+  KnownFPClass Known =
+      Info.computeKnownFPClass(SrcReg, fcNegZero | fcSubnormal);
+
+  EXPECT_EQ(~(fcNegZero | fcSubnormal), Known.getKnownFPClasses());
+  EXPECT_EQ(std::nullopt, Known.getSignBit());
+}
+
 TEST_F(AArch64GISelMITest, TestFPClassFLogPosZero) {
   StringRef MIRString = R"(
     %val:_(s32) = G_FCONSTANT float 0.0