librustc: Stop generating first-class aggregates in visit glue, since they kick us off fast isel. Closes #4352. rs=minor-perf-improvement

Author: pcwalton

src/librustc/middle/trans/common.rs

@@ -1094,6 +1094,8 @@ fn C_cstr(cx: @crate_ctxt, s: ~str) -> ValueRef {
     return g;
 }
 
+// NB: Do not use `do_spill_noroot` to make this into a constant string, or
+// you will be kicked off fast isel. See issue #4352 for an example of this.
 fn C_estr_slice(cx: @crate_ctxt, s: ~str) -> ValueRef {
     let cs = llvm::LLVMConstPointerCast(C_cstr(cx, s), T_ptr(T_i8()));
     C_struct(~[cs, C_uint(cx, str::len(s) + 1u /* +1 for null */)])

src/librustc/middle/trans/reflect.rs

@@ -46,8 +46,17 @@ impl reflector {
     }
 
     fn c_slice(s: ~str) -> ValueRef {
-        let ss = C_estr_slice(self.bcx.ccx(), s);
-        do_spill_noroot(self.bcx, ss)
+        // We're careful to not use first class aggregates here because that
+        // will kick us off fast isel. (Issue #4352.)
+        let bcx = self.bcx;
+        let str_vstore = ty::vstore_slice(ty::re_static);
+        let str_ty = ty::mk_estr(bcx.tcx(), str_vstore);
+        let scratch = scratch_datum(bcx, str_ty, false);
+        let c_str = PointerCast(bcx, C_cstr(bcx.ccx(), s), T_ptr(T_i8()));
+        Store(bcx, c_str, GEPi(bcx, scratch.val, [ 0, 0 ]));
+        let len = C_uint(bcx.ccx(), s.len() + 1);
+        Store(bcx, len, GEPi(bcx, scratch.val, [ 0, 1 ]));
+        scratch.val
     }
 
     fn c_size_and_align(t: ty::t) -> ~[ValueRef] {