From f39057d2e11ebde238777547d9d808fb9a5f5fb5 Mon Sep 17 00:00:00 2001 From: Christopher Haster Date: Thu, 9 May 2024 17:34:13 -0500 Subject: [PATCH] Forced RVO in LFSR_CAT_* macros somehow I think I'm understanding a bit more how RVO interacts with inline functions. And by that I mean I'm learning that the way RVO interacts with inline functions is unfortunately very cursed... Just take a look at this diff. This change should be a noop. But somehow it saves 200 bytes of RAM: code stack before: 33684 2824 after: 33664 (-0.1%) 2624 (-7.1%) I think what's happening is passing the result of lfsr_data_from* into lfsr_data_cat is somehow preventing RVO, because the parameter would need to be copied into the right argument slot? (argument registers?) But we really don't need a copy, because lfsr_data_cat should end up inlined. By inserting a compound literal, we force RVO, and all of these unnecessary copies get cleaned up after lfsr_data_cat is inlined. Keep in mind, in a perfect world, lfsr_data_cat should be a noop. But I could be wrong about all of this. It's not really clear what the compiler is doing, and I haven't dived that far into the disassembly... --- lfs.c | 25 +++++++++++++------------ 1 file changed, 13 insertions(+), 12 deletions(-) diff --git a/lfs.c b/lfs.c index ba5eb89e..218ed846 100644 --- a/lfs.c +++ b/lfs.c @@ -1502,12 +1502,12 @@ static inline lfs_size_t lfsr_cat_size(lfsr_cat_t cat) { #define LFSR_LLEB128_DSIZE 4 #define LFSR_CAT_LEB128(_word) \ - lfsr_data_cat( \ - lfsr_data_fromleb128(_word, (uint8_t[LFSR_LEB128_DSIZE]){0})) + lfsr_data_cat(*(lfsr_data_t[1]){ \ + lfsr_data_fromleb128(_word, (uint8_t[LFSR_LEB128_DSIZE]){0})}) #define LFSR_CAT_LLEB128(_word) \ - lfsr_data_cat( \ - lfsr_data_fromlleb128(_word, (uint8_t[LFSR_LLEB128_DSIZE]){0})) + lfsr_data_cat(*(lfsr_data_t[1]){ \ + lfsr_data_fromlleb128(_word, (uint8_t[LFSR_LLEB128_DSIZE]){0})}) static inline lfsr_data_t lfsr_data_fromleb128(uint32_t word, uint8_t buffer[static LFSR_LEB128_DSIZE]) { @@ -1830,7 +1830,7 @@ typedef struct lfsr_ecksum { #define LFSR_ECKSUM_DSIZE (4+4) #define LFSR_CAT_ECKSUM_(_ecksum, _buffer) \ - lfsr_data_cat(lfsr_data_fromecksum(_ecksum, _buffer)) + lfsr_data_cat(*(lfsr_data_t[1]){lfsr_data_fromecksum(_ecksum, _buffer)}) #define LFSR_CAT_ECKSUM(_ecksum) \ LFSR_CAT_ECKSUM_(_ecksum, (uint8_t[LFSR_ECKSUM_DSIZE]){0}) @@ -1887,7 +1887,7 @@ static int lfsr_data_readecksum(lfs_t *lfs, lfsr_data_t *data, #define LFSR_BPTR_DSIZE (4+5+4+4+4) #define LFSR_CAT_BPTR_(_bptr, _buffer) \ - lfsr_data_cat(lfsr_data_frombptr(_bptr, _buffer)) + lfsr_data_cat(*(lfsr_data_t[1]){lfsr_data_frombptr(_bptr, _buffer)}) #define LFSR_CAT_BPTR(_bptr) \ LFSR_CAT_BPTR_(_bptr, (uint8_t[LFSR_BPTR_DSIZE]){0}) @@ -2099,7 +2099,7 @@ static inline bool lfsr_grm_isrm(const lfsr_grm_t *grm, lfsr_smid_t mid) { } #define LFSR_CAT_GRM_(_grm, _buffer) \ - lfsr_data_cat(lfsr_data_fromgrm(_grm, _buffer)) + lfsr_data_cat(*(lfsr_data_t[1]){lfsr_data_fromgrm(_grm, _buffer)}) #define LFSR_CAT_GRM(_grm) \ LFSR_CAT_GRM_(_grm, (uint8_t[LFSR_GRM_DSIZE]){0}) @@ -4170,7 +4170,7 @@ static inline int lfsr_btree_cmp( #define LFSR_BRANCH_DSIZE (5+4+4) #define LFSR_CAT_BRANCH_(_branch, _buffer) \ - lfsr_data_cat(lfsr_data_frombranch(_branch, _buffer)) + lfsr_data_cat(*(lfsr_data_t[1]){lfsr_data_frombranch(_branch, _buffer)}) #define LFSR_CAT_BRANCH(_branch) \ LFSR_CAT_BRANCH_(_branch, (uint8_t[LFSR_BRANCH_DSIZE]){0}) @@ -4243,7 +4243,7 @@ static int lfsr_data_readbranch(lfs_t *lfs, lfsr_data_t *data, #define LFSR_BTREE_DSIZE (5+LFSR_BRANCH_DSIZE) #define LFSR_CAT_BTREE_(_btree, _buffer) \ - lfsr_data_cat(lfsr_data_frombtree(_btree, _buffer)) + lfsr_data_cat(*(lfsr_data_t[1]){lfsr_data_frombtree(_btree, _buffer)}) #define LFSR_CAT_BTREE(_btree) \ LFSR_CAT_BTREE_(_btree, (uint8_t[LFSR_BTREE_DSIZE]){0}) @@ -5314,7 +5314,7 @@ static inline int lfsr_shrub_cmp( #define LFSR_SHRUB_DSIZE (5+4) #define LFSR_CAT_SHRUB_(_rbyd, _buffer) \ - lfsr_data_cat(lfsr_data_fromshrub(_rbyd, _buffer)) + lfsr_data_cat(*(lfsr_data_t[1]){lfsr_data_fromshrub(_rbyd, _buffer)}) #define LFSR_CAT_SHRUB(_rbyd) \ LFSR_CAT_SHRUB_(_rbyd, (uint8_t[LFSR_SHRUB_DSIZE]){0}) @@ -5519,7 +5519,7 @@ static inline bool lfsr_mptr_ismrootanchor(const lfsr_mptr_t *mptr) { #define LFSR_MPTR_DSIZE (5+5) #define LFSR_CAT_MPTR_(_mptr, _buffer) \ - lfsr_data_cat(lfsr_data_frommptr(_mptr, _buffer)) + lfsr_data_cat(*(lfsr_data_t[1]){lfsr_data_frommptr(_mptr, _buffer)}) #define LFSR_CAT_MPTR(_mptr) \ LFSR_CAT_MPTR_(_mptr, (uint8_t[LFSR_MPTR_DSIZE]){0}) @@ -8113,7 +8113,8 @@ typedef struct lfsr_geometry { #define LFSR_GEOMETRY_DSIZE (4+5) #define LFSR_CAT_GEOMETRY_(_geometry, _buffer) \ - lfsr_data_cat(lfsr_data_fromgeometry(_geometry, _buffer)) + lfsr_data_cat(*(lfsr_data_t[1]){ \ + lfsr_data_fromgeometry(_geometry, _buffer)}) #define LFSR_CAT_GEOMETRY(_geometry) \ LFSR_CAT_GEOMETRY_(_geometry, (uint8_t[LFSR_GEOMETRY_DSIZE]){0})