asm2wasm: incorrect translation of switch for big 64 bit values

[pepyakin]

Source: rust-lang/rust#42630 (comment)

For example:

function () {
  "use asm";

  var abort=env.abort;

  function main() {
    var $0 = i64(), $x = i64();
    $x = i64_const(0,0);
    $0 = $x;
    switch (i64($0)) {
      case i64_const(0,2146435072):  {
          abort();
	  break;
      }
      default: {
        return;
      }
    }
  }
  return { main: main };
}

gets translated to

(module
 (type $FUNCSIG$v (func))
 (import "env" "abort" (func $abort))
 (import "env" "memory" (memory $0 256 256))
 (import "env" "table" (table 0 0 anyfunc))
 (import "env" "memoryBase" (global $memoryBase i32))
 (import "env" "tableBase" (global $tableBase i32))
 (export "main" (func $main))
 (func $main
  (local $$0 i64)
  (local $$x i64)
  (set_local $$x
   (i64.const 0)
  )
  (set_local $$0
   (get_local $$x)
  )
  (block $switch
   (block $switch-default
    (block $switch-case
     (br_table $switch-case $switch-default
      (i32.wrap/i64
       (i64.sub
        (get_local $$0)
        (i64.const 9218868437227405312)
       )
      )
     )
    )
    (block
     (call $abort)
     (br $switch)
    )
   )
   (return)
  )
 )
)

IIUC, i32.wrap/i64 takes i64 and trunctates it to only lower 32 bits. So in this case, result of i32.wrap/i64 will be 0, meaning first switch-case will be taken and then program aborted.

[kripken]

Thanks for reporting, proposed fix in #1111. This makes it check if the offset condition has any upper bits, in which case it branches to the default. Does the output look ok to you?

 (func $main
  (local $$0 i64)
  (local $$x i64)
  (local $2 i64)
  (set_local $$x
   (i64.const 0)
  )
  (set_local $$0
   (get_local $$x)
  )
  (block $switch
   (block $switch-default
    (block $switch-case
     (br_table $switch-case $switch-default
      (i32.wrap/i64
       (block (result i64)
        (set_local $2
         (i64.sub
          (get_local $$0)
          (i64.const 9218868437227405312)
         )
        )
        (br_if $switch-default
         (i32.wrap/i64
          (i64.shr_u
           (get_local $2)
           (i64.const 32)
          )
         )
        )
        (get_local $2)
       )
      )
     )
    )
    (block
     (call $abort)
     (br $switch)
    )
   )
   (return)
  )
 )

[pepyakin]

Looks good to me. However, I would like to validate this PR on the original case. But I'm not sure how can I build emscripten with binaryen on #1111 branch...

[kripken]

To validate, check out and build binaryen manually. Then set the env var BINARYEN to point to the the dir containing binaryen when running emcc.

[pepyakin]

Is there any way to pass this to rustc?
BINARYEN env var doesn't work (

[kripken]

Hmm, I don't know how rustc works. But if the env var fails, you can edit the ~/.emscripten file to point BINARYEN_ROOT to the dir you have binaryen. (Unless rustc customizes the location of the .emscripten file? Probably it doesn't but I'm not sure.)

[pepyakin]

Yep, that worked, thank you!
Can confirm that the original case is valid!

[kripken]

Thanks for confirming, merged.