In "symbol + ofs" addressing modes, limit the range of "ofs" in 64 bits

In 32-bit mode, a symbolic reference "symbol + ofs" (address of "symbol" plus "ofs" bytes) can always be resolved by the linker into a 32-bit quantity that fits the 32-bit displacement field of x86 addressing modes.

Not so in 64-bit mode: first, the displacement field is still 32 bits but the full address is 64 bits; second, the displacement is relative to the RIP instruction pointer.  In the "small code model" that CompCert uses for x86-64, excessively large offsets lead to link-time overflows of this 32-bit displacement field.

This commit addresses the issue by limiting the "ofs" part of "symbol + ofs" global addressing models to the range [-2^24, 2^24 - 1].  As explained in the AMD64 ELF ABI document, this is a safe range in the small code model, under the assumption that no global symbol is bigger than 2^24 bytes.  GCC seems to be using a wider range [-2^31, 2^24 - 1] but I'd rather be safe.

The limitation of the "ofs" offset is achieved by extending the mechanisms already present to ensure that "ofs" in "reg + ofs" indexed addressing modes fits in 32-bit signed:
- Op.addressing_valid checks that the "ofs" part of "symbol + ofs" addressing modes is in the correct interval;
- SelectOp.addressing turns invalid addressings into lea's + indexed addressings;
- Asmgen.normalize_addrmode_64 turns lea's with invalid addressings into simpler lea's + addq of the large offset.

1 files changed, 7 insertions, 2 deletions

diff --git a/x86/Asmgenproof1.v b/x86/Asmgenproof1.v
index aade95d2..86a5218a 100644
--- a/x86/Asmgenproof1.v
+++ b/x86/Asmgenproof1.v
@@ -495,9 +495,14 @@ Lemma normalize_addrmode_64_correct:
   | (am', Some delta) => Val.addl (eval_addrmode64 ge am' rs) (Vlong delta)
   end.
 Proof.
-  intros; destruct am as [base ofs [n|r]]; simpl; auto.
-  destruct (zeq (Int.signed (Int.repr n)) n); simpl; auto.
+  intros; destruct am as [base ofs [n|[id delta]]]; simpl.
+- destruct (offset_in_range n); auto; simpl.
   rewrite ! Val.addl_assoc. apply f_equal. apply f_equal. simpl. rewrite Int64.add_zero_l; auto.
+- destruct (ptroffset_in_range delta); auto.
+  destruct Archi.ptr64 eqn:SF; auto; simpl.
+  rewrite ! Val.addl_assoc. apply f_equal. apply f_equal.
+  rewrite <- Genv.shift_symbol_address_64 by auto.
+  f_equal. rewrite Ptrofs.add_zero_l, Ptrofs.of_int64_to_int64 by auto. auto.
 Qed.
 
 (** Processor conditions and comparisons *)