[clang] [flang] [llvm] [flang] Add runtime trampoline pool for W^X compliance (PR #183108)

[Sairudra More via cfe-commits]

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+//===-- flang-rt/runtime/trampoline.h ----------------------------*- C++-*-===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+//
+// Internal declarations for the W^X-compliant trampoline pool.
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef FLANG_RT_RUNTIME_TRAMPOLINE_H_
+#define FLANG_RT_RUNTIME_TRAMPOLINE_H_
+
+#include <cstddef>
+#include <cstdint>
+
+namespace Fortran::runtime::trampoline {
+
+/// Per-trampoline data entry. Stored in a writable (non-executable) region.
+/// Each entry is paired with a trampoline code stub in the executable region.
+struct TrampolineData {
+  const void *calleeAddress;
+  const void *staticChainAddress;
+};
+
+/// Default number of trampoline slots in the pool.
+/// Can be overridden via FLANG_TRAMPOLINE_POOL_SIZE environment variable.
+constexpr std::size_t kDefaultPoolSize = 1024;
+
+/// Size of each trampoline code stub in bytes (platform-specific).
+#if defined(__x86_64__) || defined(_M_X64)
+// x86-64 trampoline stub:
+//   movq TDATA_OFFSET(%rip), %r10    # load static chain from TDATA
+//   movabsq $0, %r11                 # placeholder for callee address
+//   jmpq *%r11
+// Actually we use an indirect approach through the TDATA pointer:
+//   movq (%r10), %r10                # load static chain (8 bytes)
+//   -- but we need the TDATA pointer first
+// Simplified approach for x86-64:
+//   leaq tdata_entry(%rip), %r11     # get TDATA entry address
+//   movq 8(%r11), %r10               # load static chain
+//   jmpq *(%r11)                     # jump to callee
+constexpr std::size_t kTrampolineStubSize = 32;
+constexpr int kNestRegister = 10; // %r10 is the nest/static chain register
+#elif defined(__aarch64__) || defined(_M_ARM64)
+// AArch64 trampoline stub:
+//   adr x17, tdata_entry             # get TDATA entry address
+//   ldr x18, [x17, #8]              # load static chain
+//   ldr x17, [x17]                  # load callee address
+//   br x17
+constexpr std::size_t kTrampolineStubSize = 32;
+constexpr int kNestRegister = 18; // x18 is the platform register
+#elif defined(__powerpc64__) || defined(__ppc64__)
+constexpr std::size_t kTrampolineStubSize = 48;
+constexpr int kNestRegister = 11; // r11
+#else
+// Fallback: generous size
+constexpr std::size_t kTrampolineStubSize = 64;
+constexpr int kNestRegister = 0;
+#endif
+
+/// Alignment requirement for trampoline code stubs.
+constexpr std::size_t kTrampolineAlignment = 16;
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Saieiei wrote:

Removed — it was unused. If we need it later we can add it back.

https://github.com/llvm/llvm-project/pull/183108