[llvm] ValueTracking: Fix handling of fadd with mixed denormal modes (PR #175454)

[Matt Arsenault via llvm-commits]

https://github.com/arsenm created https://github.com/llvm/llvm-project/pull/175454

Fix case where the input mode is IEEE, the output flushes, and the
input could be subnormal. Also improves accuracy with positive zero
case.

>From fbad7d83511a473266b4a6ca92622f1051d7d90c Mon Sep 17 00:00:00 2001
From: Matt Arsenault <Matthew.Arsenault at amd.com>
Date: Sun, 11 Jan 2026 20:53:47 +0100
Subject: [PATCH] ValueTracking: Fix handling of fadd with mixed denormal modes

Fix case where the input mode is IEEE, the output flushes, and the
input could be subnormal. Also improves accuracy with positive zero
case.
---
 llvm/lib/Analysis/ValueTracking.cpp          |  5 ++-
 llvm/test/Transforms/Attributor/nofpclass.ll | 46 +++++++++++++++++++-
 2 files changed, 49 insertions(+), 2 deletions(-)

diff --git a/llvm/lib/Analysis/ValueTracking.cpp b/llvm/lib/Analysis/ValueTracking.cpp
index dbb44c8828545..5340d41dd3683 100644
--- a/llvm/lib/Analysis/ValueTracking.cpp
+++ b/llvm/lib/Analysis/ValueTracking.cpp
@@ -5560,7 +5560,10 @@ void computeKnownFPClass(const Value *V, const APInt &DemandedElts,
         // Doubling 0 will give the same 0.
         if (SelfAdd && KnownRHS.isKnownNeverLogicalPosZero(Mode) &&
             (Mode.Output == DenormalMode::IEEE ||
-             Mode.Output == DenormalMode::PreserveSign))
+             (Mode.Output == DenormalMode::PreserveSign &&
+              KnownRHS.isKnownNeverPosSubnormal()) ||
+             (Mode.Output == DenormalMode::PositiveZero &&
+              KnownRHS.isKnownNeverSubnormal())))
           Known.knownNot(fcPosZero);
 
         // (fadd x, 0.0) is guaranteed to return +0.0, not -0.0.
diff --git a/llvm/test/Transforms/Attributor/nofpclass.ll b/llvm/test/Transforms/Attributor/nofpclass.ll
index bb0b1930ffe58..4260c4ff1115a 100644
--- a/llvm/test/Transforms/Attributor/nofpclass.ll
+++ b/llvm/test/Transforms/Attributor/nofpclass.ll
@@ -3607,7 +3607,7 @@ define float @fadd_double_no_pzero_maybe_undef(float nofpclass(pzero) %arg) {
 ; still be flushed.
 define float @fadd_double_no_zero__output_only_is_ftz(float noundef nofpclass(zero) %arg) #7 {
 ; CHECK: Function Attrs: mustprogress nofree norecurse nosync nounwind willreturn memory(none)
-; CHECK-LABEL: define noundef nofpclass(pzero) float @fadd_double_no_zero__output_only_is_ftz
+; CHECK-LABEL: define noundef float @fadd_double_no_zero__output_only_is_ftz
 ; CHECK-SAME: (float noundef nofpclass(zero) [[ARG:%.*]]) #[[ATTR13]] {
 ; CHECK-NEXT:    [[ADD:%.*]] = fadd float [[ARG]], [[ARG]]
 ; CHECK-NEXT:    ret float [[ADD]]
@@ -3675,6 +3675,50 @@ define float @assume_returned_arg(float %arg) {
   ret float %arg
 }
 
+define float @fadd_double_no_zero__output_only_is_ftpz(float noundef nofpclass(zero) %arg) #4 {
+; CHECK: Function Attrs: mustprogress nofree norecurse nosync nounwind willreturn memory(none)
+; CHECK-LABEL: define noundef nofpclass(nzero) float @fadd_double_no_zero__output_only_is_ftpz
+; CHECK-SAME: (float noundef nofpclass(zero) [[ARG:%.*]]) #[[ATTR12]] {
+; CHECK-NEXT:    [[ADD:%.*]] = fadd float [[ARG]], [[ARG]]
+; CHECK-NEXT:    ret float [[ADD]]
+;
+  %add = fadd float %arg, %arg
+  ret float %add
+}
+
+define float @fadd_double_no_zero_or_nsub__output_only_is_ftpz(float noundef nofpclass(zero nsub) %arg) #4 {
+; CHECK: Function Attrs: mustprogress nofree norecurse nosync nounwind willreturn memory(none)
+; CHECK-LABEL: define noundef nofpclass(nzero) float @fadd_double_no_zero_or_nsub__output_only_is_ftpz
+; CHECK-SAME: (float noundef nofpclass(zero nsub) [[ARG:%.*]]) #[[ATTR12]] {
+; CHECK-NEXT:    [[ADD:%.*]] = fadd float [[ARG]], [[ARG]]
+; CHECK-NEXT:    ret float [[ADD]]
+;
+  %add = fadd float %arg, %arg
+  ret float %add
+}
+
+define float @fadd_double_no_zero_or_psub__output_only_is_ftpz(float noundef nofpclass(zero psub) %arg) #4 {
+; CHECK: Function Attrs: mustprogress nofree norecurse nosync nounwind willreturn memory(none)
+; CHECK-LABEL: define noundef nofpclass(nzero) float @fadd_double_no_zero_or_psub__output_only_is_ftpz
+; CHECK-SAME: (float noundef nofpclass(zero psub) [[ARG:%.*]]) #[[ATTR12]] {
+; CHECK-NEXT:    [[ADD:%.*]] = fadd float [[ARG]], [[ARG]]
+; CHECK-NEXT:    ret float [[ADD]]
+;
+  %add = fadd float %arg, %arg
+  ret float %add
+}
+
+define float @fadd_double_no_zero_or_sub__output_only_is_ftpz(float noundef nofpclass(zero sub) %arg) #4 {
+; CHECK: Function Attrs: mustprogress nofree norecurse nosync nounwind willreturn memory(none)
+; CHECK-LABEL: define noundef nofpclass(zero) float @fadd_double_no_zero_or_sub__output_only_is_ftpz
+; CHECK-SAME: (float noundef nofpclass(zero sub) [[ARG:%.*]]) #[[ATTR12]] {
+; CHECK-NEXT:    [[ADD:%.*]] = fadd float [[ARG]], [[ARG]]
+; CHECK-NEXT:    ret float [[ADD]]
+;
+  %add = fadd float %arg, %arg
+  ret float %add
+}
+
 declare i64 @_Z13get_global_idj(i32 noundef)
 
 attributes #0 = { "denormal-fp-math"="preserve-sign,preserve-sign" }