[PATCH] D69411: [MC] Calculate difference of symbols in two fragments when possible

[Nick Desaulniers via Phabricator via llvm-commits]

nickdesaulniers added inline comments.


================
Comment at: llvm/lib/MC/MCExpr.cpp:584
+    // the two symbols belong to two fragments in the same section.
+    // FIXME: can we resolve .if conditions while finalizing layout?
+    if (IsCond && SecB.getFragmentList().getNextNode(*FragB) == FragA &&
----------------
jcai19 wrote:
> peter.smith wrote:
> > It is difficult to see how it would be possible to resolve .if conditions at layout time in a single pass assembler. In theory the assembler could evaluate all conditional blocks and select between them at layout time, if such a layout could be converged on.
> Thanks for the clarification, although I am not sure I follow.  The code looks iterative to me https://llvm.org/doxygen/MCAssembler_8cpp_source.html#l00785. I was thinking as the loop iterates and calls layoutOnce, we can relax (if needed) and calculate the sizes of the fragments before a .if statement and resolve the condition there. But I am not completely convinced by myself that it is doable. Also there some cases like the one below will create more complexity.
> 
> foo: jump to bar
> ...
> .if . - foo = ${constant integer}
> instr1
> .else.
> instr2
> .endif
> ...
> bar:
> 
> This creates a loop of dependency as depending on the instruction selected in the if-else block, the size of the jump instruction may change due to the number of bits it need to specify the offset, which in turn affects which instruction should be chosen.
> 
See also b/132538429.

> In-order to evaluate the if statement truthfully, it will have to be done after (well, during, since the result could change the relaxation) relaxation, and LLVM MC is just not set up for this currently. Even then, you could probably construct an if statement that could create a paradox, and never converge on a valid relaxation.


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https://reviews.llvm.org/D69411