improve codegen for variables that are assigned but never used. (ref #…

…9343)

this builds and passes tests with LLVM assertions, but we should be
on the lookout for regressions here.

src/codegen.cpp

@@ -1298,6 +1298,10 @@ static void simple_escape_analysis(jl_value_t *expr, bool esc, jl_codectx_t *ctx
  simple_escape_analysis(jl_exprarg(e,1), esc, ctx);
  simple_escape_analysis(jl_exprarg(e,2), esc, ctx);
  }
+ else if (e->head == assign_sym) {
+ // don't consider assignment LHS as a variable "use"
+ simple_escape_analysis(jl_exprarg(e,1), esc, ctx);
+ }
  else if (e->head != line_sym) {
  size_t elen = jl_array_dim0(e->args);
  for(i=0; i < elen; i++) {
@@ -2770,6 +2774,8 @@ static void emit_assignment(jl_value_t *l, jl_value_t *r, jl_codectx_t *ctx)
  }
  else {
  rval = emit_expr(r, ctx, true);
+ if (!vi.used) // don't actually do the assignment if the var is never read
+ return;
  // Make sure this is already boxed. If not, there was
  // something wrong in the earlier analysis as this should
  // have been alloca'd