[lld] [LLD][COFF] Add support for ARM64EC import call thunks with extended range (PR #109703)

[Jacek Caban via llvm-commits]

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+REQUIRES: aarch64, x86
+RUN: split-file %s %t.dir && cd %t.dir
+
+RUN: llvm-mc -filetype=obj -triple=arm64ec-windows test.s -o test.obj
+RUN: llvm-mc -filetype=obj -triple=arm64ec-windows %S/Inputs/loadconfig-arm64ec.s -o loadconfig-arm64ec.obj
+RUN: llvm-lib -machine:arm64ec -def:test.def -out:test.lib
+
+RUN: lld-link -machine:arm64ec -dll -noentry -out:out.dll loadconfig-arm64ec.obj test.obj test.lib
+
+RUN: llvm-objdump -d out.dll | FileCheck --check-prefix=DISASM %s
+DISASM:      0000000180001000 <.text>:
+DISASM-NEXT: 180001000: 52800000     mov     w0, #0x0                // =0
+DISASM-NEXT: 180001004: d65f03c0     ret
+DISASM-NEXT:                 ...
+DISASM-NEXT: 188001008: b000000b     adrp    x11, 0x188002000
+DISASM-NEXT: 18800100c: f940016b     ldr     x11, [x11]
+DISASM-NEXT: 188001010: f0fbffea     adrp    x10, 0x180000000
+DISASM-NEXT: 188001014: 9100014a     add     x10, x10, #0x0
+DISASM-NEXT: 188001018: 90fc0010     adrp    x16, 0x180001000 <.text>
+DISASM-NEXT: 18800101c: 91000210     add     x16, x16, #0x0
+DISASM-NEXT: 188001020: d61f0200     br      x16
+
+#--- test.s
+    .text
+    .globl __icall_helper_arm64ec
+    .p2align 2, 0x0
+__icall_helper_arm64ec:
+    mov w0, #0
+    ret
+
+    .space 0x8000000
+
+    .data
+    .rva __imp_func
----------------
cjacek wrote:

On ARM64EC, `__imp_func` references the auxiliary IAT, not the regular one. The auxiliary IAT is guaranteed to be directly callable by EC code, so the caller doesn’t need to use `__icall_helper_arm64ec`, as it would for indirect calls. Initially, the auxiliary IAT is filled with `__impchk_*` thunks (#107931), which handle the use of `__icall_helper_arm64ec` to call via the regular IAT. The OS may later update the auxiliary IAT entries to allow direct EC calls if the callee is also EC.

For ARM64EC, func references the x86 thunk that calls the regular IAT (`__imp_aux_func`), while `#func` references the ARM thunk that calls the auxiliary IAT (`__imp_func`). At runtime, if the imported function is x86, two thunks are involved: `#func` calls `__imp_func`, which points to `__impchk_func`, and this ultimately invokes the imported function pointer stored in `__imp_aux_func` using `__icall_helper_arm64ec`.

https://github.com/llvm/llvm-project/pull/109703