cc-wrapper: enable frame pointers by default.

Allow easy system-level profiling with e.g. Linux perf or bpf(trace).

Profiling applications in NixOS is quite expensive due to the size of
closures of basic but widely-used libraries. Full-system profiling is
even harder as it effectively requires re-compiling the world with ~0
caching.

By enabling it by default, we make this trivially possible (except for
the packages that explicitly specify -fomit-frame-pointers, of course)

Make an exception for 32bit x86 since it only has 8 general purpose
registers.

Frame pointers are enabled by default in Fedora, Ubuntu, and Arch Linux.

pkgs/build-support/cc-wrapper/default.nix

@@ -801,8 +801,15 @@ stdenvNoCC.mkDerivation {
       export hardening_unsupported_flags="${concatStringsSep " " ccHardeningUnsupportedFlags}"
     ''
 
+    # Do not prevent omission of framepointers on x86 32bit due to the small
+    # number of general purpose registers. Keeping EBP available provides
+    # non-trivial performance benefits.
+    + optionalString (!targetPlatform.isx86_32) ''
+      echo " -fno-omit-frame-pointer -mno-omit-leaf-frame-pointer " >> $out/nix-support/cc-cflags-before
+    ''
+
     # For clang, this is handled in add-clang-cc-cflags-before.sh
-    + lib.optionalString (!isClang && machineFlags != [ ]) ''
+    + optionalString (!isClang && machineFlags != [ ]) ''
       printf "%s\n" ${lib.escapeShellArgs machineFlags} >> $out/nix-support/cc-cflags-before
     ''