cc-wrapper: make machine configuration configurable

It is useful to make these dynamic and not bake them into gcc. This
means we don’t have to rebuild gcc to change these values. Instead, we
will pass cflags to gcc based on platform values. This was already
done hackily for android gcc (which is multi-target), but not for our
own gccs which are single target.

To accomplish this, we need to add a few things:

- add ‘arch’ to cpu
- add NIX_CFLAGS_COMPILE_BEFORE flag (goes before args)
- set -march everywhere
- set mcpu, mfpu, mmode, and mtune based on targetPlatform.gcc flags

cc-wrapper: only set -march when it is in the cpu type

Some architectures don’t have a good mapping of -march. For instance
POWER architecture doesn’t support the -march flag at all!

https://gcc.gnu.org/onlinedocs/gcc/RS_002f6000-and-PowerPC-Options.html#RS_002f6000-and-PowerPC-Options

3 files changed, 43 insertions, 11 deletions

diff --git a/pkgs/build-support/cc-wrapper/add-flags.sh b/pkgs/build-support/cc-wrapper/add-flags.sh
index 9762894607ac..1358b167f6ec 100644
--- a/pkgs/build-support/cc-wrapper/add-flags.sh
+++ b/pkgs/build-support/cc-wrapper/add-flags.sh
@@ -6,6 +6,7 @@
 
 var_templates_list=(
     NIX+CFLAGS_COMPILE
+    NIX+CFLAGS_COMPILE_BEFORE
     NIX+CFLAGS_LINK
     NIX+CXXSTDLIB_COMPILE
     NIX+CXXSTDLIB_LINK
@@ -43,5 +44,9 @@ if [ -e @out@/nix-support/cc-ldflags ]; then
     NIX_@infixSalt@_LDFLAGS+=" $(< @out@/nix-support/cc-ldflags)"
 fi
 
+if [ -e @out@/nix-support/cc-cflags-before ]; then
+    NIX_@infixSalt@_CFLAGS_COMPILE_BEFORE="$(< @out@/nix-support/cc-cflags-before) $NIX_@infixSalt@_CFLAGS_COMPILE_BEFORE"
+fi
+
 # That way forked processes will not extend these environment variables again.
 export NIX_CC_WRAPPER_@infixSalt@_FLAGS_SET=1
diff --git a/pkgs/build-support/cc-wrapper/cc-wrapper.sh b/pkgs/build-support/cc-wrapper/cc-wrapper.sh
index 8003fe1d8f38..bb7890100087 100644
--- a/pkgs/build-support/cc-wrapper/cc-wrapper.sh
+++ b/pkgs/build-support/cc-wrapper/cc-wrapper.sh
@@ -135,7 +135,7 @@ source @out@/nix-support/add-hardening.sh
 
 # Add the flags for the C compiler proper.
 extraAfter=($NIX_@infixSalt@_CFLAGS_COMPILE)
-extraBefore=(${hardeningCFlags[@]+"${hardeningCFlags[@]}"})
+extraBefore=(${hardeningCFlags[@]+"${hardeningCFlags[@]}"} $NIX_@infixSalt@_CFLAGS_COMPILE_BEFORE)
 
 if [ "$dontLink" != 1 ]; then
 
diff --git a/pkgs/build-support/cc-wrapper/default.nix b/pkgs/build-support/cc-wrapper/default.nix
index 9569c6e78c8a..4d4dbc640b28 100644
--- a/pkgs/build-support/cc-wrapper/default.nix
+++ b/pkgs/build-support/cc-wrapper/default.nix
@@ -279,24 +279,51 @@ stdenv.mkDerivation {
       export hardening_unsupported_flags="${builtins.concatStringsSep " " (cc.hardeningUnsupportedFlags or [])}"
     ''
 
-    + optionalString hostPlatform.isCygwin ''
-      hardening_unsupported_flags+=" pic"
-    ''
+    # Machine flags. These are necessary to support
 
-    + optionalString targetPlatform.isMinGW ''
-      hardening_unsupported_flags+=" stackprotector"
+    # TODO: We should make a way to support miscellaneous machine
+    # flags and other gcc flags as well.
+
+    # Always add -march based on cpu in triple. Sometimes there is a
+    # discrepency (x86_64 vs. x86-64), so we provide an "arch" arg in
+    # that case.
+    + optionalString (targetPlatform ? platform.gcc.arch || targetPlatform.parsed.cpu ? arch) ''
+      echo "-march=${targetPlatform.platform.gcc.arch or targetPlatform.parsed.cpu.arch}" >> $out/nix-support/cc-cflags-before
     ''
 
-    + optionalString targetPlatform.isAvr ''
-      hardening_unsupported_flags+=" stackprotector pic"
+    # -mcpu is not very useful. You should use mtune and march
+    # instead. It’s provided here for backwards compatibility.
+    + optionalString (targetPlatform ? platform.gcc.cpu) ''
+      echo "-mcpu=${targetPlatform.platform.gcc.cpu}" >> $out/nix-support/cc-cflags-before
     ''
 
-    + optionalString targetPlatform.isNetBSD ''
-      hardening_unsupported_flags+=" stackprotector fortify"
+    # -mfloat-abi only matters on arm32 but we set it here
+    # unconditionally just in case. If the abi specifically sets hard
+    # vs. soft floats we use it here.
+    + optionalString (targetPlatform ? platform.gcc.float-abi || targetPlatform.parsed.abi ? float) ''
+      echo "-mfloat-abi=${targetPlatform.platform.gcc.float-abi or targetPlatform.parsed.abi.float}" >> $out/nix-support/cc-cflags-before
+    ''
+    + optionalString (targetPlatform ? platform.gcc.fpu) ''
+      echo "-mfpu=${targetPlatform.platform.gcc.fpu}" >> $out/nix-support/cc-cflags-before
+    ''
+    + optionalString (targetPlatform ? platform.gcc.mode) ''
+      echo "-mmode=${targetPlatform.platform.gcc.mode}" >> $out/nix-support/cc-cflags-before
+    ''
+    + optionalString (targetPlatform ? platform.gcc.tune) ''
+      echo "-mtune=${targetPlatform.platform.gcc.tune}" >> $out/nix-support/cc-cflags-before
     ''
 
-    + optionalString (targetPlatform.libc == "newlib") ''
+    # TODO: categorize these and figure out a better place for them
+    + optionalString hostPlatform.isCygwin ''
+      hardening_unsupported_flags+=" pic"
+    '' + optionalString targetPlatform.isMinGW ''
+      hardening_unsupported_flags+=" stackprotector"
+    '' + optionalString targetPlatform.isAvr ''
+      hardening_unsupported_flags+=" stackprotector pic"
+    '' + optionalString (targetPlatform.libc == "newlib") ''
       hardening_unsupported_flags+=" stackprotector fortify pie pic"
+    '' + optionalString targetPlatform.isNetBSD ''
+      hardening_unsupported_flags+=" stackprotector fortify"
     ''
 
     + optionalString (libc != null && targetPlatform.isAvr) ''