Auto merge of #957 - christianpoveda:ptr-align-offset, r=RalfJung

Fixes for align_offset

This addresses @RalfJung's comments in https://github.com/rust-lang/miri/pull/945

src/shims/mod.rs

@@ -1,10 +1,10 @@
+pub mod dlsym;
+pub mod env;
 pub mod foreign_items;
 pub mod intrinsics;
 pub mod tls;
-pub mod dlsym;
-pub mod env;
 
-use rustc::{ty, mir};
+use rustc::{mir, ty};
 
 use crate::*;
 
@@ -18,7 +18,11 @@ fn find_fn(
         ret: Option<mir::BasicBlock>,
     ) -> InterpResult<'tcx, Option<&'mir mir::Body<'tcx>>> {
         let this = self.eval_context_mut();
-        trace!("eval_fn_call: {:#?}, {:?}", instance, dest.map(|place| *place));
+        trace!(
+            "eval_fn_call: {:#?}, {:?}",
+            instance,
+            dest.map(|place| *place)
+        );
 
         // First, run the common hooks also supported by CTFE.
         if this.hook_fn(instance, args, dest)? {
@@ -27,27 +31,10 @@ fn find_fn(
         }
         // There are some more lang items we want to hook that CTFE does not hook (yet).
         if this.tcx.lang_items().align_offset_fn() == Some(instance.def.def_id()) {
-
-            let n = {
-                let ptr = this.force_ptr(this.read_scalar(args[0])?.not_undef()?)?;
-                let align = this.force_bits(
-                    this.read_scalar(args[1])?.not_undef()?,
-                    this.pointer_size()
-                )? as usize;
-
-                let stride = this.memory().get(ptr.alloc_id)?.align.bytes() as usize;
-                // if the allocation alignment is at least the required alignment, we use the
-                // libcore implementation
-                if stride >= align {
-                    ((stride + ptr.offset.bytes() as usize) as *const ())
-                        .align_offset(align) as u128
-                } else {
-                    u128::max_value()
-                }
-            };
-
             let dest = dest.unwrap();
-            let n = this.truncate(n, dest.layout);
+            let n = this
+                .align_offset(args[0], args[1])?
+                .unwrap_or_else(|| this.truncate(u128::max_value(), dest.layout));
             this.write_scalar(Scalar::from_uint(n, dest.layout.size), dest)?;
             this.goto_block(ret)?;
             return Ok(None);
@@ -65,4 +52,39 @@ fn find_fn(
         // Otherwise, load the MIR.
         Ok(Some(this.load_mir(instance.def, None)?))
     }
+
+    fn align_offset(
+        &mut self,
+        ptr_op: OpTy<'tcx, Tag>,
+        align_op: OpTy<'tcx, Tag>,
+    ) -> InterpResult<'tcx, Option<u128>> {
+        let this = self.eval_context_mut();
+
+        let req_align = this.force_bits(
+            this.read_scalar(align_op)?.not_undef()?,
+            this.pointer_size(),
+        )? as usize;
+
+        // FIXME: This should actually panic in the interpreted program
+        if !req_align.is_power_of_two() {
+            throw_unsup_format!("Required alignment should always be a power of two")
+        }
+
+        let ptr_scalar = this.read_scalar(ptr_op)?.not_undef()?;
+
+        if let Ok(ptr) = this.force_ptr(ptr_scalar) {
+            let cur_align = this.memory().get(ptr.alloc_id)?.align.bytes() as usize;
+            if cur_align >= req_align {
+                // if the allocation alignment is at least the required alignment we use the
+                // libcore implementation
+                return Ok(Some(
+                    (this.force_bits(ptr_scalar, this.pointer_size())? as *const i8)
+                        .align_offset(req_align) as u128,
+                ));
+            }
+        }
+        // If the allocation alignment is smaller than then required alignment or the pointer was
+        // actually an integer, we return `None`
+        Ok(None)
+    }
 }

tests/run-pass/aligned_utf8_check.rs

@@ -1,13 +1,6 @@
 fn main() {
     const N: usize = 10;
-
-    let x = vec![0x4141u16; N];
-
-    let mut y: Vec<u8> = unsafe { std::mem::transmute(x) };
-    unsafe { y.set_len(2 * N) };
-
-    println!("{:?}", std::str::from_utf8(&y).unwrap());
-
-    let mut x: Vec<u16> = unsafe { std::mem::transmute(y) };
-    unsafe { x.set_len(N) };
+    let vec = vec![0x4141414141414141u64; N];
+    let content = unsafe { std::slice::from_raw_parts(vec.as_ptr() as *const u8, 8 * N) };
+    println!("{:?}", std::str::from_utf8(content).unwrap());
 }

tests/run-pass/aligned_utf8_check.stdout

@@ -1 +1 @@
-"AAAAAAAAAAAAAAAAAAAA"
+"AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA"