From 783301298f1e151b82c1be21bc6a214b9ef525bd Mon Sep 17 00:00:00 2001
From: Nikita Popov <npopov@redhat.com>
Date: Thu, 20 Oct 2022 12:28:31 +0200
Subject: [PATCH] Don't use usub.with.overflow intrinsic

The canonical form of a usub.with.overflow check in LLVM are
separate sub + icmp instructions, rather than a usub.with.overflow
intrinsic. Using usub.with.overflow will generally result in worse
optimization potential.

The backend will attempt to form usub.with.overflow when it comes
to actual instruction selection. This is not fully reliable, but
I believe this is a better tradeoff than using the intrinsic in
IR.

Fixes #103285.
---
 compiler/rustc_codegen_llvm/src/builder.rs       | 13 ++++++++-----
 .../issue-103285-ptr-addr-overflow-check.rs      | 16 ++++++++++++++++
 2 files changed, 24 insertions(+), 5 deletions(-)
 create mode 100644 src/test/codegen/issue-103285-ptr-addr-overflow-check.rs

diff --git a/compiler/rustc_codegen_llvm/src/builder.rs b/compiler/rustc_codegen_llvm/src/builder.rs
index fca43a0d86ddd..9cb36ce7f1890 100644
--- a/compiler/rustc_codegen_llvm/src/builder.rs
+++ b/compiler/rustc_codegen_llvm/src/builder.rs
@@ -365,11 +365,14 @@ impl<'a, 'll, 'tcx> BuilderMethods<'a, 'tcx> for Builder<'a, 'll, 'tcx> {
                 Int(I64) => "llvm.ssub.with.overflow.i64",
                 Int(I128) => "llvm.ssub.with.overflow.i128",
 
-                Uint(U8) => "llvm.usub.with.overflow.i8",
-                Uint(U16) => "llvm.usub.with.overflow.i16",
-                Uint(U32) => "llvm.usub.with.overflow.i32",
-                Uint(U64) => "llvm.usub.with.overflow.i64",
-                Uint(U128) => "llvm.usub.with.overflow.i128",
+                Uint(_) => {
+                    // Emit sub and icmp instead of llvm.usub.with.overflow. LLVM considers these
+                    // to be the canonical form. It will attempt to reform llvm.usub.with.overflow
+                    // in the backend if profitable.
+                    let sub = self.sub(lhs, rhs);
+                    let cmp = self.icmp(IntPredicate::IntULT, lhs, rhs);
+                    return (sub, cmp);
+                }
 
                 _ => unreachable!(),
             },
diff --git a/src/test/codegen/issue-103285-ptr-addr-overflow-check.rs b/src/test/codegen/issue-103285-ptr-addr-overflow-check.rs
new file mode 100644
index 0000000000000..a3499babea21b
--- /dev/null
+++ b/src/test/codegen/issue-103285-ptr-addr-overflow-check.rs
@@ -0,0 +1,16 @@
+// compile-flags: -O -C debug-assertions=yes
+
+#![crate_type = "lib"]
+#![feature(strict_provenance)]
+
+#[no_mangle]
+pub fn test(src: *const u8, dst: *const u8) -> usize {
+    // CHECK-LABEL: @test(
+    // CHECK-NOT: panic
+    let src_usize = src.addr();
+    let dst_usize = dst.addr();
+    if src_usize > dst_usize {
+        return src_usize - dst_usize;
+    }
+    return 0;
+}