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[MC][ELF] Improve st_size propagation rule 

`.symver foo, foo@ver` creates the MCSymbolELF `foo@ver` whose almost all
attributes (including st_size) should inherit from `foo` (GNU as behavior).

a041ef1bd8 added st_size propagation which works
for many cases but fails for the following one:

```
.set __GLIBC_2_12_sys_errlist, _sys_errlist_internal
.type   __GLIBC_2_12_sys_errlist,@object
.size   __GLIBC_2_12_sys_errlist, 1080
.symver __GLIBC_2_12_sys_errlist, sys_errlist@GLIBC_2.12
...
_sys_errlist_internal:
.size   _sys_errlist_internal, 1072
```

`sys_errlist@GLIBC_2.12`'s st_size is 1072 (incorrect), which does not match
`__GLIBC_2_12_sys_errlist`'s st_size: 1080.

The problem is that `Base` is (the final) `_sys_errlist_internal` while we want
to respect (the intermediate) `__GLIBC_2_12_sys_errlist`'s st_size.
Fix this by following the MCSymbolRefExpr assignment chain and finding
the closest non-null `getSize()`, which covers most needs. Notably MCBinaryExpr
is not handled, but it is rare enough to matter.

Reviewed By: peter.smith

Differential Revision: https://reviews.llvm.org/D123283
This commit is contained in:
Fangrui Song 2022-04-08 10:06:43 -07:00
parent 67acc34e2a
commit e07dfa5328
2 changed files with 24 additions and 5 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

20
llvm/lib/MC/ELFObjectWriter.cpp
View File

@ -547,9 +547,27 @@ void ELFWriter::writeSymbol(SymbolTableWriter &Writer, uint32_t StringIndex,
uint64_t Size = 0;
const MCExpr *ESize = MSD.Symbol->getSize();
if (!ESize && Base)
if (!ESize && Base) {
// For expressions like .set y, x+1, if y's size is unset, inherit from x.
ESize = Base->getSize();
// For `.size x, 2; y = x; .size y, 1; z = y; z1 = z; .symver y, y@v1`, z,
// z1, and y@v1's st_size equals y's. However, `Base` is `x` which will give
// us 2. Follow the MCSymbolRefExpr assignment chain, which covers most
// needs. MCBinaryExpr is not handled.
const MCSymbolELF *Sym = &Symbol;
while (Sym->isVariable()) {
if (auto *Expr =
dyn_cast<MCSymbolRefExpr>(Sym->getVariableValue(false))) {
Sym = cast<MCSymbolELF>(&Expr->getSymbol());
if (!Sym->getSize())
continue;
ESize = Sym->getSize();
}
break;
}
}
if (ESize) {
int64_t Res;
if (!ESize->evaluateKnownAbsolute(Res, Layout))

9
llvm/test/MC/ELF/offset.s
View File

@ -14,15 +14,15 @@
# CHECK-NEXT: 000000000000000d 42 OBJECT GLOBAL DEFAULT [[#A]] d1
# CHECK-NEXT: 000000000000000d 42 OBJECT GLOBAL DEFAULT [[#A]] d2
# CHECK-NEXT: 0000000000000001 41 OBJECT GLOBAL DEFAULT [[#A]] e
# CHECK-NEXT: 0000000000000001 42 OBJECT GLOBAL DEFAULT [[#A]] e1
# CHECK-NEXT: 0000000000000001 42 OBJECT GLOBAL DEFAULT [[#A]] e2
# CHECK-NEXT: 0000000000000001 41 OBJECT GLOBAL DEFAULT [[#A]] e1
# CHECK-NEXT: 0000000000000001 41 OBJECT GLOBAL DEFAULT [[#A]] e2
# CHECK-NEXT: 0000000000000002 42 OBJECT GLOBAL DEFAULT [[#A]] e3
# CHECK-NEXT: 0000000000000005 0 NOTYPE GLOBAL DEFAULT [[#A]] test2_a
# CHECK-NEXT: 0000000000000005 0 NOTYPE GLOBAL DEFAULT [[#A]] test2_b
# CHECK-NEXT: 0000000000000009 0 NOTYPE GLOBAL DEFAULT [[#A]] test2_c
# CHECK-NEXT: 0000000000000009 0 NOTYPE GLOBAL DEFAULT [[#A]] test2_d
# CHECK-NEXT: 0000000000000004 0 NOTYPE GLOBAL DEFAULT ABS test2_e
# CHECK-NEXT: 0000000000000001 42 OBJECT GLOBAL DEFAULT [[#A]] e@v1
# CHECK-NEXT: 0000000000000001 41 OBJECT GLOBAL DEFAULT [[#A]] e@v1
.data
@ -45,7 +45,8 @@ d2 = d1
e = a + (1 - 1)
.size e, 41
## FIXME These st_size fields inherit from e instead of a.
## These st_size fields inherit from e instead of a.
## TODO e3's st_size should inherit from e.
.set e1, e
.set e2, e1
e3 = e1 + 1