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rv64: use pc-relative addressing for globals

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When extern was added, the rv64 code switched regular globals from
using la (a pseudo-instruction that uses pc-relative addressing
with auipc + addi) to lui + addi (absolute addressing). Presumably,
this was done to match gcc's output with -fPIC and -fno-PIC
respectively.  However, the key difference here is actually the
directive `%option pic` vs `%option nopic`, which determines whether
la uses the GOT or not.

This broke linking as PIE (for instance in compilers with
--enable-default-pie), even when there are no global references
outside the executable. The other architectures always use pc-relative
addressing, so rv64 should do the same.

It turns out there are more specific pseudo-instructions lla and
lga we can use instead. lla uses auipc+addi, and lga uses auipc+ld
from the GOT.

lga does not allow an offset. If you try to use one, it will assemble
without error, but fail at link time with

(.text+0x10): dangerous relocation: The addend isn't allowed for R_RISCV_GOT_HI20

For now, just add a check for this. A proper fix involves some
changes to rv64/isel like is done for amd64.
This commit is contained in:
Michael Forney 2026-06-02 10:57:06 -07:00 committed by Quentin Carbonneaux
parent 80d745c842
commit e786f06032
1 changed files with 6 additions and 8 deletions
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14
rv64/emit.c
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@ -133,8 +133,12 @@ emitaddr(Con *c, FILE *f)
{
assert((c->sym.type & ~SExt) == SGlo);
fputs(str(c->sym.id), f);
if (c->bits.i)
if (c->bits.i) {
/* TODO: fix isel to ensure no offset for SGlo */
if (c->sym.type & SExt)
die("extern with offset is not supported");
fprintf(f, "+%"PRIi64, c->bits.i);
}
}
static void
@ -233,14 +237,8 @@ loadaddr(Con *c, char *rn, FILE *f)
switch (c->sym.type) {
case SGlo:
fprintf(f, "\tlui %s, %%hi(", rn);
emitaddr(c, f);
fprintf(f, ")\n\taddi %s, %s, %%lo(", rn, rn);
emitaddr(c, f);
fputs(")\n", f);
break;
case SExt:
fprintf(f, "\tla %s, ", rn);
fprintf(f, "\t%s %s, ", c->sym.type == SExt ? "lga" : "lla", rn);
emitaddr(c, f);
fputc('\n', f);
break;