Cheng Shao pushed to branch wip/grow-block-free at Glasgow Haskell Compiler / GHC Commits: 4e7adb8c by Cheng Shao at 2025-12-23T19:59:40+01:00 rts: opportunistically grow the MutableByteArray# in-place in resizeMutableByteArray# Following !15234, this patch improves `resizeMutableByteArray#` memory efficiency by growing the `MutableByteArray#` in-place if possible, addressing an old todo comment here. - - - - - 1 changed file: - rts/PrimOps.cmm Changes: ===================================== rts/PrimOps.cmm ===================================== @@ -258,20 +258,35 @@ stg_shrinkMutableByteArrayzh ( gcptr mba, W_ new_size ) stg_resizzeMutableByteArrayzh ( gcptr mba, W_ new_size ) // MutableByteArray# s -> Int# -> State# s -> (# State# s,MutableByteArray# s #) { + W_ old_size, old_wds, new_wds, new_free; + W_ bd; + ASSERT(new_size `ge` 0); - if (new_size <= StgArrBytes_bytes(mba)) { + old_size = StgArrBytes_bytes(mba); + if (new_size <= old_size) { call stg_shrinkMutableByteArrayzh(mba, new_size); return (mba); + } + + bd = Bdescr(mba); + old_wds = BYTES_TO_WDS(SIZEOF_StgArrBytes) + ROUNDUP_BYTES_TO_WDS(old_size); + new_wds = BYTES_TO_WDS(SIZEOF_StgArrBytes) + ROUNDUP_BYTES_TO_WDS(new_size); + new_free = mba + WDS(new_wds); + + // Just like stg_shrinkMutableByteArrayzh above, we try to grow mba + // in-place if possible. The conditions are similar to the + // conditions when we can set bd->free when shrinking mba, and we + // also need to check that we don't grow past the end of current block. + if (bdescr_free(bd) == mba + WDS(old_wds) && + (bd == StgRegTable_rCurrentAlloc(BaseReg) || bd == Capability_pinned_object_block(MyCapability())) && + new_free <= bdescr_start(bd) + (TO_W_(bdescr_blocks(bd)) * BLOCK_SIZE)) { + bdescr_free(bd) = new_free; + StgArrBytes_bytes(mba) = new_size; + return (mba); } else { (P_ new_mba) = call stg_newByteArrayzh(new_size); - // maybe at some point in the future we may be able to grow the - // MBA in-place w/o copying if we know the space after the - // current MBA is still available, as often we want to grow the - // MBA shortly after we allocated the original MBA. So maybe no - // further allocations have occurred by then. - // copy over old content prim %memcpy(BYTE_ARR_CTS(new_mba), BYTE_ARR_CTS(mba), StgArrBytes_bytes(mba), SIZEOF_W); View it on GitLab: https://gitlab.haskell.org/ghc/ghc/-/commit/4e7adb8cf245cabc9bce7e766b6b5661... -- View it on GitLab: https://gitlab.haskell.org/ghc/ghc/-/commit/4e7adb8cf245cabc9bce7e766b6b5661... You're receiving this email because of your account on gitlab.haskell.org.