From d3ef80147b02fc64795769348b46ad7ac8980dda Mon Sep 17 00:00:00 2001
From: Benjamin Bouvier <public@benj.me>
Date: Tue, 1 Oct 2019 17:21:52 +0200
Subject: [PATCH] [meta] Simplify handling of variable arguments in
 legalization;

If a legalization contains varargs, it defers creating the binding for
the varargs, making the generated code easier to understand.
---
 cranelift/codegen/meta/src/gen_legalizer.rs | 27 +++++++++++----------
 1 file changed, 14 insertions(+), 13 deletions(-)

diff --git a/cranelift/codegen/meta/src/gen_legalizer.rs b/cranelift/codegen/meta/src/gen_legalizer.rs
index d71f97b10d..13c4351fb4 100644
--- a/cranelift/codegen/meta/src/gen_legalizer.rs
+++ b/cranelift/codegen/meta/src/gen_legalizer.rs
@@ -44,6 +44,10 @@ fn unwrap_inst(
         .args
         .iter()
         .enumerate()
+        .filter(|(arg_num, _)| {
+            // Variable args are specially handled after extracting args.
+            !inst.operands_in[*arg_num].is_varargs()
+        })
         .map(|(arg_num, arg)| match &arg {
             Expr::Var(var_index) => var_pool.get(*var_index).name.as_ref(),
             Expr::Literal(_) => {
@@ -122,16 +126,9 @@ fn unwrap_inst(
                 } else if op.is_value() {
                     let n = inst.value_opnums.iter().position(|&i| i == op_num).unwrap();
                     fmtln!(fmt, "pos.func.dfg.resolve_aliases(args[{}]),", n);
-                } else if op.is_varargs() {
-                    let n = inst.imm_opnums.iter().chain(inst.value_opnums.iter()).max().map(|n| n + 1).unwrap_or(0);
-                    // We need to create a `Vec` here, as using a slice would result in a borrowck
-                    // error later on.
-                    fmtln!(fmt, "\
-                        args.iter().skip({}).map(|&arg| pos.func.dfg.resolve_aliases(arg)).collect::<Vec<_>>(),\
-                    ", n);
                 } else {
-                    // This is a value list argument.
-                    assert!(iform.has_value_list);
+                    // This is a value list argument or a varargs.
+                    assert!(iform.has_value_list || op.is_varargs());
                 }
             };
 
@@ -167,10 +164,14 @@ fn unwrap_inst(
             let name = &var_pool
                 .get(apply.args[i].maybe_var().expect("vararg without name"))
                 .name;
-
-            // Above name is set to an `Vec` representing the varargs. However it is expected to be
-            // `&[Value]` below, so we borrow it.
-            fmtln!(fmt, "let {} = &{};", name, name);
+            let n = inst
+                .imm_opnums
+                .iter()
+                .chain(inst.value_opnums.iter())
+                .max()
+                .copied()
+                .unwrap_or(0);
+            fmtln!(fmt, "let {} = &Vec::from(&args[{}..]);", name, n);
         }
     }