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w6l+selfhost: BSS optimisation — trim trailing .data zeros from filesz

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Scan the consolidated .data buffer (post-relocation) for trailing zero
bytes; set the R+W PT_LOAD's p_filesz to exclude them while p_memsz
covers the full region. The loader zero-fills the gap, so behaviour is
unchanged. Saves up to a page per binary on programs whose globals are
zero-init.

Mirrored in selfhost/cmd/w6l/out.ww so test 992's byte-identity diff
still holds. Dynamic-link path is untouched — it still errors on any
mutable global; that's the next feature.
This commit is contained in:
Hojun-Cho
2026-05-12 13:44:27 +09:00
parent 548547a1d0
commit 2480f4c272
4 changed files with 142 additions and 16 deletions
Download Patch File Download Diff File Expand all files Collapse all files

21
cmd/w6l/out.c
View File

@@ -63,11 +63,24 @@ l_emit_elf(Lnk *l, FILE *f, u64 base, u64 entry)
const u64 rx_end = text_off + l->textlen;
const int has_data = (l->datalen > 0);
/* BSS optimisation: trailing zero bytes in .data can be left out
* of the file. The loader zero-fills the gap between p_filesz and
* p_memsz, so this shrinks the binary without changing semantics.
* Scan after l_relocate has applied DATAR patches — anything still
* zero at the tail genuinely is zero-init. */
u64 bsslen = 0;
if (has_data) {
while (bsslen < l->datalen
&& l->data[l->datalen - 1 - bsslen] == 0)
bsslen++;
}
const u64 data_file_len = l->datalen - bsslen;
/* data goes at the next page boundary so the loader can grant a
* fresh page of R+W permissions without overlapping the R+X mapping. */
const u64 data_off = has_data ? ((rx_end + page - 1) & ~(page - 1)) : 0;
const u64 data_va = has_data ? (base + data_off) : 0;
const u64 file_end = has_data ? (data_off + l->datalen) : rx_end;
const u64 file_end = has_data ? (data_off + data_file_len) : rx_end;
(void)data_va;
Ehdr eh = {0};
@@ -104,7 +117,7 @@ l_emit_elf(Lnk *l, FILE *f, u64 base, u64 entry)
phw.p_offset = data_off;
phw.p_vaddr = base + data_off;
phw.p_paddr = base + data_off;
phw.p_filesz = l->datalen;
phw.p_filesz = data_file_len;
phw.p_memsz = l->datalen;
phw.p_align = page;
}
@@ -119,11 +132,11 @@ l_emit_elf(Lnk *l, FILE *f, u64 base, u64 entry)
if (l->textlen) fwrite(l->text, 1, l->textlen, f);
if (has_data) {
if (has_data && data_file_len > 0) {
/* pad to data_off */
here = ftell(f);
for (long i = here; i < (long)data_off; i++) fputc(0, f);
fwrite(l->data, 1, l->datalen, f);
fwrite(l->data, 1, data_file_len, f);
}
(void)file_end;

27
selfhost/cmd/w6l/main.combined.ww
View File

@@ -2398,6 +2398,21 @@ export fn emitelf(l: *lnk, fd: i32, base: u64, entry: u64) i32 = {
dataoff = (rxend + PAGE_SZ - 1u64) & ~(PAGE_SZ - 1u64);
};
// BSS optimisation: trailing zero bytes in .data can be left
// out of the file. The loader zero-fills the gap between
// p_filesz and p_memsz. Scan after l_relocate has applied any
// DATAR patches — anything still zero at the tail genuinely is
// zero-init. Matches cmd/w6l/out.c byte-for-byte.
let bsslen: u64 = 0u64;
if (hasdata) {
for (bsslen < l.datalen) {
let b: u8 = l.data[l.datalen - 1u64 - bsslen];
if (b != 0u8) { break; };
bsslen += 1u64;
};
};
let datafilelen: u64 = l.datalen - bsslen;
// One contiguous header buffer covering [0..0x1000), then .text.
let hdr: *u8 = os.alloc(TEXT_OFF): *u8; // zero-initialised by mmap
@@ -2441,7 +2456,7 @@ export fn emitelf(l: *lnk, fd: i32, base: u64, entry: u64) i32 = {
wru64(hdr, 128u64, dataoff); // p_offset
wru64(hdr, 136u64, base + dataoff); // p_vaddr
wru64(hdr, 144u64, base + dataoff); // p_paddr
wru64(hdr, 152u64, l.datalen); // p_filesz
wru64(hdr, 152u64, datafilelen); // p_filesz
wru64(hdr, 160u64, l.datalen); // p_memsz
wru64(hdr, 168u64, PAGE_SZ); // p_align
};
@@ -2463,8 +2478,10 @@ export fn emitelf(l: *lnk, fd: i32, base: u64, entry: u64) i32 = {
};
if (n2 != l.textlen: i64) { return -1; };
};
if (hasdata) {
// Pad to the page-aligned data offset, then write .data.
if (hasdata && datafilelen > 0u64) {
// Pad to the page-aligned data offset, then write only
// the non-zero prefix of .data. The rest is BSS — the
// loader zero-fills from p_filesz to p_memsz.
let here: u64 = TEXT_OFF + l.textlen;
let zero: u8 = 0u8;
for (here < dataoff) {
@@ -2475,13 +2492,13 @@ export fn emitelf(l: *lnk, fd: i32, base: u64, entry: u64) i32 = {
};
here += 1u64;
};
let r4: (i64 | os.oserror) = os.writeall(fd, l.data, l.datalen);
let r4: (i64 | os.oserror) = os.writeall(fd, l.data, datafilelen);
let n4: i64 = 0i64;
match (r4) {
case let v: i64 => n4 = v;
case let e: os.oserror => return -1;
};
if (n4 != l.datalen: i64) { return -1; };
if (n4 != datafilelen: i64) { return -1; };
};
return 0;
};

27
selfhost/cmd/w6l/out.ww
View File

@@ -63,6 +63,21 @@ export fn emitelf(l: *lnk, fd: i32, base: u64, entry: u64) i32 = {
dataoff = (rxend + PAGE_SZ - 1u64) & ~(PAGE_SZ - 1u64);
};
// BSS optimisation: trailing zero bytes in .data can be left
// out of the file. The loader zero-fills the gap between
// p_filesz and p_memsz. Scan after l_relocate has applied any
// DATAR patches — anything still zero at the tail genuinely is
// zero-init. Matches cmd/w6l/out.c byte-for-byte.
let bsslen: u64 = 0u64;
if (hasdata) {
for (bsslen < l.datalen) {
let b: u8 = l.data[l.datalen - 1u64 - bsslen];
if (b != 0u8) { break; };
bsslen += 1u64;
};
};
let datafilelen: u64 = l.datalen - bsslen;
// One contiguous header buffer covering [0..0x1000), then .text.
let hdr: *u8 = os.alloc(TEXT_OFF): *u8; // zero-initialised by mmap
@@ -106,7 +121,7 @@ export fn emitelf(l: *lnk, fd: i32, base: u64, entry: u64) i32 = {
wru64(hdr, 128u64, dataoff); // p_offset
wru64(hdr, 136u64, base + dataoff); // p_vaddr
wru64(hdr, 144u64, base + dataoff); // p_paddr
wru64(hdr, 152u64, l.datalen); // p_filesz
wru64(hdr, 152u64, datafilelen); // p_filesz
wru64(hdr, 160u64, l.datalen); // p_memsz
wru64(hdr, 168u64, PAGE_SZ); // p_align
};
@@ -128,8 +143,10 @@ export fn emitelf(l: *lnk, fd: i32, base: u64, entry: u64) i32 = {
};
if (n2 != l.textlen: i64) { return -1; };
};
if (hasdata) {
// Pad to the page-aligned data offset, then write .data.
if (hasdata && datafilelen > 0u64) {
// Pad to the page-aligned data offset, then write only
// the non-zero prefix of .data. The rest is BSS — the
// loader zero-fills from p_filesz to p_memsz.
let here: u64 = TEXT_OFF + l.textlen;
let zero: u8 = 0u8;
for (here < dataoff) {
@@ -140,13 +157,13 @@ export fn emitelf(l: *lnk, fd: i32, base: u64, entry: u64) i32 = {
};
here += 1u64;
};
let r4: (i64 | os.oserror) = os.writeall(fd, l.data, l.datalen);
let r4: (i64 | os.oserror) = os.writeall(fd, l.data, datafilelen);
let n4: i64 = 0i64;
match (r4) {
case let v: i64 => n4 = v;
case let e: os.oserror => return -1;
};
if (n4 != l.datalen: i64) { return -1; };
if (n4 != datafilelen: i64) { return -1; };
};
return 0;
};

83
test/wcc/620_data_link.c
View File

@@ -239,6 +239,84 @@ test_no_data_single_load(const char *bin)
return rc;
}
/* Test 5: BSS optimisation — trailing zero bytes in .data are dropped
* from the file (p_filesz < p_memsz) and the loader zero-fills the gap.
* Asserts that a trailing zero-init slot doesn't bloat the binary. */
static int
test_bss_filesz(const char *bin)
{
char exe[64];
snprintf(exe, sizeof exe, "/tmp/wwt_dl_%d.x5", getpid());
/* Two DATAW slots: a 1-byte non-zero followed by a 31-byte zero
* run. Memsz must cover all 32B; filesz should stop after the 1B
* non-zero (so file_data == 1, memsz_data == 32). */
const char *body =
"TEXT _start,$0\n"
"\tMOVQ\tnz(SB), DI\n"
"\tMOVQ\t$60, AX\n"
"\tSYSCALL\n"
"DATAW nz(SB),\"\\x07\\x00\\x00\\x00\\x00\\x00\\x00\\x00\"\n"
"DATAW zlong(SB),\"\\x00\\x00\\x00\\x00\\x00\\x00\\x00\\x00"
"\\x00\\x00\\x00\\x00\\x00\\x00\\x00\\x00"
"\\x00\\x00\\x00\\x00\\x00\\x00\\x00\\x00\"\n";
if (build(bin, body, exe) != 0) {
fprintf(stderr, "test_bss_filesz: build failed\n");
return -1;
}
uint8_t *buf = NULL;
size_t n = 0;
if (slurp(exe, &buf, &n) < 0) {
fprintf(stderr, "test_bss_filesz: cannot read exe\n");
unlink(exe);
return -1;
}
int rc = 0;
const Ehdr *eh = (const Ehdr *)buf;
const Phdr *ph = (const Phdr *)(buf + eh->e_phoff);
int saw_rw = 0;
for (int i = 0; i < eh->e_phnum; i++) {
if (ph[i].p_type != PT_LOAD) continue;
if (ph[i].p_flags != (PF_R | PF_W)) continue;
saw_rw = 1;
if (ph[i].p_filesz >= ph[i].p_memsz) {
fprintf(stderr, "test_bss_filesz: filesz=%lu, "
"memsz=%lu (want filesz<memsz)\n",
(unsigned long)ph[i].p_filesz,
(unsigned long)ph[i].p_memsz);
rc = -1;
}
/* The scan trims every trailing zero — it doesn't know
* symbol boundaries. nz is \x07 then 7 zero bytes, so
* the file ends at byte 1 (the 0x07). Memsz still covers
* all 32B and the loader zero-fills. */
if (ph[i].p_filesz != 1) {
fprintf(stderr, "test_bss_filesz: filesz=%lu, "
"want 1\n", (unsigned long)ph[i].p_filesz);
rc = -1;
}
if (ph[i].p_memsz != 32) {
fprintf(stderr, "test_bss_filesz: memsz=%lu, "
"want 32\n", (unsigned long)ph[i].p_memsz);
rc = -1;
}
}
if (!saw_rw) {
fprintf(stderr, "test_bss_filesz: no R+W PT_LOAD found\n");
rc = -1;
}
free(buf);
if (rc != 0) { unlink(exe); return rc; }
/* Behaviour check: the zero-init slot must still read as zero
* at runtime — the loader has to zero-fill the BSS gap. */
int er = run_exit(exe);
unlink(exe);
if (er != 7) {
fprintf(stderr, "test_bss_filesz: exit=%d, want 7\n", er);
return -1;
}
return 0;
}
int
main(void)
{
@@ -249,10 +327,11 @@ main(void)
if (test_write(bin) != 0) fail++;
if (test_two_loads(bin) != 0) fail++;
if (test_no_data_single_load(bin) != 0) fail++;
if (test_bss_filesz(bin) != 0) fail++;
if (fail) {
fprintf(stderr, "data_link: %d/4 subtests failed\n", fail);
fprintf(stderr, "data_link: %d/5 subtests failed\n", fail);
return 1;
}
printf("data_link: 4/4 ok\n");
printf("data_link: 5/5 ok\n");
return 0;
}