[PATCH] D24128: [ELF] PR30221 - linker script expression parser does not accept '~'

[Rui Ueyama via llvm-commits]

ruiu added inline comments.

================
Comment at: ELF/LinkerScript.cpp:1248
@@ +1247,3 @@
+  if (Tok == "~") {
+    Expr E = readParenExpr();
+    return [=](uint64_t Dot) { return ~E(Dot); };
----------------
grimar wrote:
> ruiu wrote:
> > Even if this is what gold does (I didn't test it myself), this looks very weird. If it's true, it seems a gold's bug. I think any primary expression should be allowed after `~`.
> My consern about gold/ld is that users of lld can write some script using our syntax and have a problems if they decide to switch to gold/ld for some reason. I think it is always better to have script files fully compatible if it is possible.
> Though it is always more convinent to write ~XXX then ~(XXX), therefore my position here is wonky.
I tried 

  echo 'SECTIONS { foo = ~0; }' > x
  echo 'int main() {}' | /ssd/clang/bin/clang -xc - -o /dev/null -fuse-ld=gold -Wl,--script=./x

and gold didn't complain the absence of () after ~. I believe you made a mistake.

~ operator is defined in gold's yyscript.y as follows.

  exp:
          '(' exp ')'
            { $$ = $2; }
        | '-' exp %prec UNARY
            { $$ = script_exp_unary_minus($2); }
        | '!' exp %prec UNARY
            { $$ = script_exp_unary_logical_not($2); }
        | '~' exp %prec UNARY
            { $$ = script_exp_unary_bitwise_not($2); }
        | '+' exp %prec UNARY
            { $$ = $2; }
        | exp '*' exp
            { $$ = script_exp_binary_mult($1, $3); }
        | ...

As you can see, `exp` is expected after ~.

Finally, it just doesn't make sense to require `(` only after `~` unary operator while any expression is allowed for other unary operators. If you design a language, you'd never do that.


Repository:
  rL LLVM

https://reviews.llvm.org/D24128