[RISCV] Allow delayed decision for ADD/SUB relocations

For a label difference `A-B` in assembly, if A and B are separated by a
linker-relaxable instruction, we should emit a pair of ADD/SUB
relocations (e.g. R_RISCV_ADD32/R_RISCV_SUB32,
R_RISCV_ADD64/R_RISCV_SUB64).

However, the decision is made upfront at parsing time with inadequate
heuristics (`requiresFixup`). As a result, LLVM integrated assembler
incorrectly suppresses R_RISCV_ADD32/R_RISCV_SUB32 for the following
code:
```
// Simplified from a workaround https://android-review.googlesource.com/c/platform/art/+/2619609
// Both end and begin are not defined yet. We decide ADD/SUB relocations upfront and don't know they will be needed.
.4byte end-begin

begin:
  call foo
end:
```

To fix the bug, make two primary changes:

* Delete `requiresFixups` and the overridden emitValueImpl (from D103539).
  This deletion requires accurate evaluateAsAbolute (D153097).
* In MCAssembler::evaluateFixup, call handleAddSubRelocations to emit
  ADD/SUB relocations.

However, there is a remaining issue in
MCExpr.cpp:AttemptToFoldSymbolOffsetDifference. With MCAsmLayout, we may
incorrectly fold A-B even when A and B are separated by a
linker-relaxable instruction. This deficiency is acknowledged (see
D153097), but was previously bypassed by eagerly emitting ADD/SUB using
`requiresFixups`. To address this, we partially reintroduce `canFold` (from
D61584, removed by D103539).

Some expressions (e.g. .size and .fill) need to take the `MCAsmLayout`
code path in AttemptToFoldSymbolOffsetDifference, avoiding relocations
(weird, but matching GNU assembler and needed to match user
expectation). Switch to evaluateKnownAbsolute to leverage the `InSet`
condition.

As a bonus, this change allows for the removal of some relocations for
the FDE `address_range` field in the .eh_frame section.

riscv64-64b-pcrel.s contains the main test.
Add a linker relaxable instruction to dwarf-riscv-relocs.ll to test what
it intends to test.
Merge fixups-relax-diff.ll into fixups-diff.ll.

Reviewed By: kito-cheng

Differential Revision: https://reviews.llvm.org/D155357

1 files changed, 8 insertions, 1 deletions

diff --git a/llvm/lib/MC/MCExpr.cpp b/llvm/lib/MC/MCExpr.cpp
index f2caadd..a7b9805 100644
--- a/llvm/lib/MC/MCExpr.cpp
+++ b/llvm/lib/MC/MCExpr.cpp
@@ -628,7 +628,14 @@ static void AttemptToFoldSymbolOffsetDifference(
   if ((&SecA != &SecB) && !Addrs)
     return;
 
-  if (Layout) {
+  // When layout is available, we can generally compute the difference using the
+  // getSymbolOffset path, which also avoids the possible slow fragment walk.
+  // However, linker relaxation may cause incorrect fold of A-B if A and B are
+  // separated by a linker-relaxable instruction. If the section contains
+  // instructions and InSet is false (not expressions in directive like
+  // .size/.fill), disable the fast path.
+  if (Layout && (InSet || !SecA.hasInstructions() ||
+                 !Asm->getContext().getTargetTriple().isRISCV())) {
     // If both symbols are in the same fragment, return the difference of their
     // offsets. canGetFragmentOffset(FA) may be false.
     if (FA == FB && !SA.isVariable() && !SB.isVariable()) {