[clang] [clang] Generate appropriate assume in presence of libc's memcpy (PR #119704)

[via cfe-commits]

https://github.com/serge-sans-paille created https://github.com/llvm/llvm-project/pull/119704

It is an undefined behavior to pass null arguments as memcpy source or destination parameter, so generate the appropriate llvm.assume call to communicate this to the backend.

Don't do it for builtin memcpy as they don't suffer the same behavior in case of null size.

>From 9d013f37df124fde0ed637d4b979de5ab01cbc22 Mon Sep 17 00:00:00 2001
From: serge-sans-paille <sguelton at mozilla.com>
Date: Thu, 12 Dec 2024 14:45:31 +0100
Subject: [PATCH] [clang] Generate appropriate assume in presence of libc's
 memcpy

It is an undefined behavior to pass null arguments as memcpy source or
destination parameter, so generate the appropriate llvm.assume call to
communicate this to the backend.

Don't do it for builtin memcpy as they don't suffer the same behavior in
case of null size.
---
 clang/lib/CodeGen/CGBuiltin.cpp           | 6 ++++++
 clang/test/CodeGen/catch-undef-behavior.c | 4 ++++
 2 files changed, 10 insertions(+)

diff --git a/clang/lib/CodeGen/CGBuiltin.cpp b/clang/lib/CodeGen/CGBuiltin.cpp
index 497a0b3952af8c..50cd8529120d36 100644
--- a/clang/lib/CodeGen/CGBuiltin.cpp
+++ b/clang/lib/CodeGen/CGBuiltin.cpp
@@ -4531,6 +4531,12 @@ RValue CodeGenFunction::EmitBuiltinExpr(const GlobalDecl GD, unsigned BuiltinID,
     Value *SizeVal = EmitScalarExpr(E->getArg(2));
     EmitArgCheck(TCK_Store, Dest, E->getArg(0), 0);
     EmitArgCheck(TCK_Load, Src, E->getArg(1), 1);
+    if (BuiltinID == Builtin::BImemcpy || BuiltinID == Builtin::BImempcpy) {
+      Builder.CreateAssumption(
+          Builder.CreateIsNotNull(Dest.emitRawPointer(*this)));
+      Builder.CreateAssumption(
+          Builder.CreateIsNotNull(Src.emitRawPointer(*this)));
+    }
     Builder.CreateMemCpy(Dest, Src, SizeVal, false);
     if (BuiltinID == Builtin::BImempcpy ||
         BuiltinID == Builtin::BI__builtin_mempcpy)
diff --git a/clang/test/CodeGen/catch-undef-behavior.c b/clang/test/CodeGen/catch-undef-behavior.c
index 7580290b0b0333..7ef7c78f1cebb7 100644
--- a/clang/test/CodeGen/catch-undef-behavior.c
+++ b/clang/test/CodeGen/catch-undef-behavior.c
@@ -371,6 +371,10 @@ void call_memcpy_nonnull(void *p, void *q, int sz) {
   // CHECK-TRAP: call void @llvm.ubsantrap(i8 16)
   // CHECK-COMMON-NOT: call
 
+  // CHECK-COMMON: %5 = icmp ne ptr %0, null
+  // CHECK-COMMON: call void @llvm.assume(i1 %5)
+  // CHECK-COMMON: %6 = icmp ne ptr %0, null
+  // CHECK-COMMON: call void @llvm.assume(i1 %6)
   // CHECK-COMMON: call void @llvm.memcpy.p0.p0.i64(ptr align 1 %0, ptr align 1 %1, i64 %conv, i1 false)
   memcpy(p, q, sz);
 }