[PATCH] D147983: [lld][driver] Add `--relax-gp`/`no-relax-gp` flags for GNU compatibility

[Paul Kirth via Phabricator via llvm-commits]

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GNU LD accepts `--relax-gp/--no-relax-gp` to control GP relaxation.

Since `gp` (`x3`) is being defined as the platform register in the
RISC-V psABI, LLD needs to be able to handle flags that disable gp
relaxation. The use of `gp` as the shadow call stack register is a prime
example of when a compiler aught to set such a flag for the linker, so
that linker relaxation won't conflict with the platform's usage of the
register.

This patch only adds the flag for compatibility, since LLD does not yet
implement GP relaxation, which will not conflict with a platform's usage
of the RISC-V GP register.


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D147983

Files:
  lld/ELF/Options.td
  lld/test/ELF/ignore-relax-gp.test


Index: lld/test/ELF/ignore-relax-gp.test
===================================================================
--- /dev/null
+++ lld/test/ELF/ignore-relax-gp.test
@@ -0,0 +1,3 @@
+RUN: not ld.lld --relax-gp 2>&1 | FileCheck %s
+RUN: not ld.lld --no-relax-gp 2>&1 | FileCheck %s
+CHECK: no input files
Index: lld/ELF/Options.td
===================================================================
--- lld/ELF/Options.td
+++ lld/ELF/Options.td
@@ -355,6 +355,11 @@
   "Enable target-specific relaxations if supported (default)",
   "Disable target-specific relaxations">;
 
+// It is likely that we will eventually need to implement these
+defm relax_gp: BB<"relax-gp",
+  "Ignored for compatibility with GNU linkers",
+  "Ignored for compatibility with GNU linkers">;
+
 defm reproduce:
   EEq<"reproduce",
      "Write tar file containing inputs and command to reproduce link">;


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