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

[Nikita Popov via llvm-commits]

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

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.

The transform as implemented was doubly wrong, because it just truncated the original base pointer to the index width, losing the top bits entirely. Make sure we preserve the bits and use wrapping arithmetic within the low bits.

>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 177027b8f3c5db940cb857b5f2426d695a1b1fa1 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..f874d3d5e7318 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 the 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