[llvm] [ConstantFolding] Fix handling of index width != pointer width (PR #130608)

[Nikita Popov via llvm-commits]

https://github.com/nikic updated https://github.com/llvm/llvm-project/pull/130608

>From c5ce591577e1226147c34da24c85eb852428b4d0 Mon Sep 17 00:00:00 2001
From: Nikita Popov <npopov at redhat.com>
Date: Mon, 10 Mar 2025 14:58:48 +0100
Subject: [PATCH 1/2] Add test

---
 .../ConstProp/inttoptr-gep-index-width.ll          | 14 ++++++++++++++
 1 file changed, 14 insertions(+)
 create mode 100644 llvm/test/Transforms/InstSimplify/ConstProp/inttoptr-gep-index-width.ll

diff --git a/llvm/test/Transforms/InstSimplify/ConstProp/inttoptr-gep-index-width.ll b/llvm/test/Transforms/InstSimplify/ConstProp/inttoptr-gep-index-width.ll
new file mode 100644
index 0000000000000..9f462db684e2d
--- /dev/null
+++ b/llvm/test/Transforms/InstSimplify/ConstProp/inttoptr-gep-index-width.ll
@@ -0,0 +1,14 @@
+; NOTE: Assertions have been autogenerated by utils/update_test_checks.py UTC_ARGS: --version 5
+; RUN: opt -S -passes=instsimplify < %s | FileCheck %s
+
+target datalayout = "p:16:16:16:8"
+
+; The GEP should only modify the low 8 bits of the pointer.
+define ptr @test() {
+; CHECK-LABEL: define ptr @test() {
+; CHECK-NEXT:    ret ptr null
+;
+  %base = inttoptr i16 -1 to ptr
+  %gep = getelementptr i8, ptr %base, i8 1
+  ret ptr %gep
+}

>From 1be2e4239a9da242fc23f03a3e1c4895e16bb0ac Mon Sep 17 00:00:00 2001
From: Nikita Popov <npopov at redhat.com>
Date: Mon, 10 Mar 2025 14:56:22 +0100
Subject: [PATCH 2/2] [ConstantFolding] Fix handling of index width != pointer
 width
MIME-Version: 1.0
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: 8bit

Per LangRef:

> The offsets are then added to the low bits of the base address up to the index type width, with silently-wrapping two’s complement arithmetic. If the pointer size is larger than the index size, this means that the bits outside the index type width will not be affected.
---
 llvm/lib/Analysis/ConstantFolding.cpp                    | 9 ++++++---
 .../InstSimplify/ConstProp/inttoptr-gep-index-width.ll   | 2 +-
 2 files changed, 7 insertions(+), 4 deletions(-)

diff --git a/llvm/lib/Analysis/ConstantFolding.cpp b/llvm/lib/Analysis/ConstantFolding.cpp
index d645bf8f7b621..b0ba25c3c16ac 100644
--- a/llvm/lib/Analysis/ConstantFolding.cpp
+++ b/llvm/lib/Analysis/ConstantFolding.cpp
@@ -943,18 +943,21 @@ Constant *SymbolicallyEvaluateGEP(const GEPOperator *GEP,
 
   // If the base value for this address is a literal integer value, fold the
   // getelementptr to the resulting integer value casted to the pointer type.
-  APInt BasePtr(BitWidth, 0);
+  APInt BasePtr(DL.getPointerTypeSizeInBits(Ptr->getType()), 0);
   if (auto *CE = dyn_cast<ConstantExpr>(Ptr)) {
     if (CE->getOpcode() == Instruction::IntToPtr) {
       if (auto *Base = dyn_cast<ConstantInt>(CE->getOperand(0)))
-        BasePtr = Base->getValue().zextOrTrunc(BitWidth);
+        BasePtr = Base->getValue().zextOrTrunc(BasePtr.getBitWidth());
     }
   }
 
   auto *PTy = cast<PointerType>(Ptr->getType());
   if ((Ptr->isNullValue() || BasePtr != 0) &&
       !DL.isNonIntegralPointerType(PTy)) {
-    Constant *C = ConstantInt::get(Ptr->getContext(), Offset + BasePtr);
+    // If the index size is smaller than the pointer size, add to the low
+    // bits only.
+    BasePtr.insertBits(BasePtr.trunc(BitWidth) + Offset, 0);
+    Constant *C = ConstantInt::get(Ptr->getContext(), BasePtr);
     return ConstantExpr::getIntToPtr(C, ResTy);
   }
 
diff --git a/llvm/test/Transforms/InstSimplify/ConstProp/inttoptr-gep-index-width.ll b/llvm/test/Transforms/InstSimplify/ConstProp/inttoptr-gep-index-width.ll
index 9f462db684e2d..03056e8361e21 100644
--- a/llvm/test/Transforms/InstSimplify/ConstProp/inttoptr-gep-index-width.ll
+++ b/llvm/test/Transforms/InstSimplify/ConstProp/inttoptr-gep-index-width.ll
@@ -6,7 +6,7 @@ target datalayout = "p:16:16:16:8"
 ; The GEP should only modify the low 8 bits of the pointer.
 define ptr @test() {
 ; CHECK-LABEL: define ptr @test() {
-; CHECK-NEXT:    ret ptr null
+; CHECK-NEXT:    ret ptr inttoptr (i16 -256 to ptr)
 ;
   %base = inttoptr i16 -1 to ptr
   %gep = getelementptr i8, ptr %base, i8 1