[llvm] [LLVM][AsmWriter] Fix ConstantFP zeroinitializer check (PR #196097)

[Paul Walker via llvm-commits]

https://github.com/paulwalker-arm created https://github.com/llvm/llvm-project/pull/196097

It turns out ppc_fp128 has a value where isPosZero() returns true but isNullValue() returns false.

I wondered if DoubleAPFloat might be wrong but when trying to "fix" it I found more and more places that do not care about the second double, so I reverted to this localised fix.  The change to `Constant::isNullValue()` is not required, but given the comment I figured it was worth keeping the related code consistent.

>From b9aa87b7d5b717bb05a049b6da800a256faf273f Mon Sep 17 00:00:00 2001
From: Paul Walker <paul.walker at arm.com>
Date: Wed, 6 May 2026 12:39:24 +0000
Subject: [PATCH 1/2] Add test showing bogus asm printing.

---
 llvm/test/Assembler/constant-splat.ll | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/llvm/test/Assembler/constant-splat.ll b/llvm/test/Assembler/constant-splat.ll
index 82e25adda0e10..a91468766fa73 100644
--- a/llvm/test/Assembler/constant-splat.ll
+++ b/llvm/test/Assembler/constant-splat.ll
@@ -36,6 +36,9 @@
 ; CHECK: @constant.splat.ppc_fp128 = constant <1 x ppc_fp128> splat (ppc_fp128 0xM80000000000000000000000000000000)
 @constant.splat.ppc_fp128 = constant <1 x ppc_fp128> splat (ppc_fp128 0xM80000000000000000000000000000000)
 
+; CHECK: @constant.splat.ppc_fp128.zero.1 = constant <1 x ppc_fp128> zeroinitializer
+ at constant.splat.ppc_fp128.zero.1 = constant <1 x ppc_fp128> splat (ppc_fp128 0xM00000000000000000000000000000001)
+
 ; CHECK: @constant.splat.global.ptr = constant <4 x ptr> <ptr @my_global, ptr @my_global, ptr @my_global, ptr @my_global>
 @constant.splat.global.ptr = constant <4 x ptr> splat (ptr @my_global)
 

>From 2e09fb42c71a1b25db65690abc6276d73370d9a5 Mon Sep 17 00:00:00 2001
From: Paul Walker <paul.walker at arm.com>
Date: Wed, 6 May 2026 13:02:30 +0000
Subject: [PATCH 2/2] [LLVM][AsmWriter] Fix ConstantFP zeroinitializer check.

It turns out ppc_fp128 has a value where isPosZero returns true but
isNullValue returns false.
---
 llvm/lib/IR/AsmWriter.cpp             | 2 +-
 llvm/lib/IR/Constants.cpp             | 5 ++---
 llvm/test/Assembler/constant-splat.ll | 2 +-
 3 files changed, 4 insertions(+), 5 deletions(-)

diff --git a/llvm/lib/IR/AsmWriter.cpp b/llvm/lib/IR/AsmWriter.cpp
index ab1e7c5dca6e3..e209b47a855df 100644
--- a/llvm/lib/IR/AsmWriter.cpp
+++ b/llvm/lib/IR/AsmWriter.cpp
@@ -1672,7 +1672,7 @@ static void writeConstantInternal(raw_ostream &Out, const Constant *CV,
     Type *Ty = CFP->getType();
 
     if (Ty->isVectorTy()) {
-      if (CFP->isPosZero()) {
+      if (CFP->getValue().bitcastToAPInt().isZero()) {
         Out << "zeroinitializer";
         return;
       }
diff --git a/llvm/lib/IR/Constants.cpp b/llvm/lib/IR/Constants.cpp
index ed30b5dac51fd..f0c9ee0b774fe 100644
--- a/llvm/lib/IR/Constants.cpp
+++ b/llvm/lib/IR/Constants.cpp
@@ -96,11 +96,10 @@ bool Constant::isNullValue() const {
   if (const ConstantByte *CB = dyn_cast<ConstantByte>(this))
     return CB->isZero();
 
-  // +0.0 is null.
   if (const ConstantFP *CFP = dyn_cast<ConstantFP>(this))
     // ppc_fp128 determine isZero using high order double only
-    // Should check the bitwise value to make sure all bits are zero.
-    return CFP->isExactlyValue(+0.0);
+    // so check the bitwise value to make sure all bits are zero.
+    return CFP->getValue().bitcastToAPInt().isZero();
 
   // constant zero is zero for aggregates, cpnull is null for pointers, none for
   // tokens.
diff --git a/llvm/test/Assembler/constant-splat.ll b/llvm/test/Assembler/constant-splat.ll
index a91468766fa73..c4c335b684ebd 100644
--- a/llvm/test/Assembler/constant-splat.ll
+++ b/llvm/test/Assembler/constant-splat.ll
@@ -36,7 +36,7 @@
 ; CHECK: @constant.splat.ppc_fp128 = constant <1 x ppc_fp128> splat (ppc_fp128 0xM80000000000000000000000000000000)
 @constant.splat.ppc_fp128 = constant <1 x ppc_fp128> splat (ppc_fp128 0xM80000000000000000000000000000000)
 
-; CHECK: @constant.splat.ppc_fp128.zero.1 = constant <1 x ppc_fp128> zeroinitializer
+; CHECK: @constant.splat.ppc_fp128.zero.1 = constant <1 x ppc_fp128> splat (ppc_fp128 0xM00000000000000000000000000000001)
 @constant.splat.ppc_fp128.zero.1 = constant <1 x ppc_fp128> splat (ppc_fp128 0xM00000000000000000000000000000001)
 
 ; CHECK: @constant.splat.global.ptr = constant <4 x ptr> <ptr @my_global, ptr @my_global, ptr @my_global, ptr @my_global>