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[LLD] Evaluation of DEFINED linker script directive produces wrong output #64600

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edward-jones opened this issue Aug 10, 2023 · 3 comments · Fixed by #65866
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[LLD] Evaluation of DEFINED linker script directive produces wrong output #64600

edward-jones opened this issue Aug 10, 2023 · 3 comments · Fixed by #65866
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@MaskRay
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@edward-jones
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The following linker script causes unexpected behaviour when APPLE is not defined by any input objects. The unexpected output is that APPLE will resolve to '1' instead of '2'.

APPLE = DEFINED (APPLE) ? 1 : 2;

It appears that when the symbol is first processed it is added to the symbol table with the expected value, however LLD reevaluates symbol assignments, and during these subsequent evaluations of the expression the DEFINED now returns 1 as APPLE has been inserted in the symbol table.
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llvmbot commented Aug 10, 2023

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@renesas-kyle-finch
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I have run into a similar issue. I think it has to do with when the symbol being set is the same symbol being checked by DEFINED.

Building off the original example when APPLE is not defined:

Scenario 1:
APPLE = DEFINED (APPLE) ? APPLE : 2;
This evaluates to 0 instead of 2

Scenario 2:
ORANGE = DEFINED (APPLE) ? APPLE : 2;
This correctly evaluates to 2

@MaskRay MaskRay self-assigned this Sep 8, 2023
@MaskRay MaskRay mentioned this issue Sep 9, 2023
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MaskRay commented Sep 9, 2023

Created #65866

ZijunZhaoCCK pushed a commit to ZijunZhaoCCK/llvm-project that referenced this issue Sep 19, 2023
@MaskRay @ZijunZhaoCCK
[ELF] Respect orders of symbol assignments and DEFINED (llvm#65866)
f55be80 
Fix llvm#64600: the currently implementation is minimal (see
https://reviews.llvm.org/D83758), and an assignment like
`__TEXT_REGION_ORIGIN__ = DEFINED(__TEXT_REGION_ORIGIN__) ? __TEXT_REGION_ORIGIN__ : 0;`
(used by avr-ld[1]) leads to a value of zero (default value in `declareSymbol`),
which is unexpected.

Assign orders to symbol assignments and references so that
for a script-defined symbol, the `DEFINED` results match users'
expectation. I am unclear about GNU ld's exact behavior, but this hopefully
matches its behavior in the majority of cases.

[1]: https://sourceware.org/git/?p=binutils-gdb.git;a=blob;f=ld/scripttempl/avr.sc
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[ELF] Respect orders of symbol assignments and DEFINED MaskRay/llvm-project
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@MaskRay @edward-jones @llvmbot @renesas-kyle-finch