[llvm] [RISCV][CFI] add function epilogue cfi information (PR #110810)

[Sam Elliott via llvm-commits]

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@@ -51,10 +56,15 @@ define void @test_scoped_alloca(i64 %n) {
 ; RV64-NEXT:    call use_addr
 ; RV64-NEXT:    mv sp, s1
 ; RV64-NEXT:    addi sp, s0, -32
+; RV64-NEXT:    .cfi_def_cfa sp, 32
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lenary wrote:

This function, for example, looks correct:
- The prolog sets a CFA offset (i recall, from `sp`?)
- Then it says where a bunch of registers have been saved, relative to that CFA
- Then the `.cfi_def_cfa s0, 0` directive, i believe, says "now, to calculate the CFA, use `s0+0`"
- Most of the function happens
- You then get to the epilog and say "now calculate the CFA as `sp+0`" using this line
- You then restore the value of `ra`, `s0`, and `s1`

This means if a debugger stops between the `ld s0, 16(sp)` and the `ret`, then asking where some variables are kept should not use a calculation based on the restored `s0`, it should now be based on `sp`.

https://github.com/llvm/llvm-project/pull/110810