Cheng Shao pushed to branch wip/rts-fix-missing-untag at Glasgow Haskell Compiler / GHC

Commits:

• e2a53b7d
  by Cheng Shao at 2026-07-14T07:11:54+00:00

  rts: fix missing UNTAG in stg_readTVarIOzh
  
  This patch fixes missing UNTAG on the current value closure read from
  StgTVar. UNTAG is a no-op when it's stg_TREC_HEADER_info which is word
  aligned; it may be a tagged closure, and reading info table from the
  tagged address is an unaligned load which may cause issues on
  platforms with strict alignment requirements.
  
  Co-authored-by: Codex <codex@openai.com>
  

• 4ec65891
  by Cheng Shao at 2026-07-14T07:13:02+00:00

  rts: fix missing UNTAG in stg_control0zh_ll
  
  This patch fixes missing UNTAG on the cont closure returned by
  captureContinuationAndAbort. In case it's not NULL,
  captureContinuationAndAbort returns a tagged StgContinuation closure,
  in which case it must be untagged before accessing the
  apply_mask_frame field.
  
  In the past it worked out of luck: when apply_mask_frame was NULL then
  mask_frame_offset is also 0 so the control flow didn't diverge to a
  wrong path. Still, this is horribly wrong and will crash once
  StgContinuation struct is refactored and fields are shuffled around.
  
  Co-authored-by: Codex <codex@openai.com>
  

2 changed files:

• rts/ContinuationOps.cmm
• rts/PrimOps.cmm

Changes:

   }

   W_ apply_mask_frame;

-  apply_mask_frame = StgContinuation_apply_mask_frame(cont);

+  apply_mask_frame = StgContinuation_apply_mask_frame(UNTAG(cont));

   // The stack has been updated, so it’s time to apply the input function,

   // passing the captured continuation and a RealWorld token as arguments.

 again:

     result = %acquire StgTVar_current_value(tvar);

-    resultinfo = %INFO_PTR(result);

+    // when result is stg_TREC_HEADER_info, it's word aligned so UNTAG

+    // is no-op; if it's a tagged closure we must UNTAG it to avoid an

+    // unaligned memory access

+    resultinfo = %INFO_PTR(UNTAG(result));

     if (resultinfo == stg_TREC_HEADER_info) {

         goto again;

     }