Stefan Schulze Frielinghaus pushed to branch wip/fix-gen_workspace at Glasgow Haskell Compiler / GHC

Commits:

• 16b5251e
  by Stefan Schulze Frielinghaus at 2025-09-16T10:59:49+02:00

  rts: Fix alignment for gen_workspace #26334
  
  After a0fa4941903272c48b050d24e93eec819eff51bd bootstrap is broken on
  s390x and errors out with
  
  rts/sm/GCThread.h:207:5: error:
       error: alignment of array elements is greater than element size
        207 |     gen_workspace gens[];
            |     ^~~~~~~~~~~~~
  
  The alignment constraint is applied via the attribute to the type
  gen_workspace and leaves the underlying type struct gen_workspace_
  untouched.  On Aarch64, x86, and s390x the struct has a size of 128
  bytes.  On Aarch64 and x86 the alignments of 128 and 64 are divisors of
  the size, respectively, which is why the type is a viable member type
  for an array.  However, on s390x, the alignment is 256 and therefore is
  not a divisor of the size and hence cannot be used for arrays.
  
  Basically I see two fixes here.  Either decrease the alignment
  requirement on s390x, or by applying the alignment constraint on the
  struct itself.  The former might affect performance as noted in
  a0fa4941903272c48b050d24e93eec819eff51bd.  The latter introduces padding
  bits whenever necessary in order to ensure that
  sizeof(gen_workspace[N])==N*sizeof(gen_workspace) holds which is done by
  this patch.
  

1 changed file:

• rts/sm/GCThread.h

Changes:

 // platforms.

 #define GEN_WORKSPACE_ALIGNMENT CACHELINE_SIZE

-typedef struct gen_workspace_ {

+typedef struct ATTRIBUTE_ALIGNED(GEN_WORKSPACE_ALIGNMENT) gen_workspace_ {

     generation * gen;           // the gen for this workspace

     struct gc_thread_ * my_gct; // the gc_thread that contains this workspace

     bdescr *     part_list;

     StgWord      n_part_blocks;      // count of above

     StgWord      n_part_words;

-} gen_workspace ATTRIBUTE_ALIGNED(GEN_WORKSPACE_ALIGNMENT);

+} gen_workspace;

 /* ----------------------------------------------------------------------------

    GC thread object