From bf57132888478ead1b64434616bd832ada6b3c03 Mon Sep 17 00:00:00 2001
From: Michael Bishop <cleverca22@gmail.com>
Date: Fri, 5 Jun 2020 23:34:38 -0300
Subject: [PATCH] gcc: Fix MinGW exception handling so Rust works

reasoning:
sjlj (short jump long jump) exception handling makes no sense on x86_64, it's forcably slowing programs down as it produces a constant overhead. On x86_64 we have SEH (Structured Exception Handling) and we should use that. On i686, we do not have SEH, and have to use sjlj with dwarf2. Hence it's now conditional on x86_32

(cherry picked from commit e27e475f0d7292f1ecb64c4f40de2d417b46f45a)
(cherry picked from commit 58ffaee5d7d02058a1dd8644990b2f3b6c8a94ef)
---
 .../compilers/gcc/common/configure-flags.nix    | 17 ++++++++++++++---
 1 file changed, 14 insertions(+), 3 deletions(-)

diff --git a/pkgs/development/compilers/gcc/common/configure-flags.nix b/pkgs/development/compilers/gcc/common/configure-flags.nix
index aa0fcb5062894..1a2947e84a81f 100644
--- a/pkgs/development/compilers/gcc/common/configure-flags.nix
+++ b/pkgs/development/compilers/gcc/common/configure-flags.nix
@@ -24,6 +24,16 @@
 assert cloog != null -> stdenv.lib.versionOlder version "5";
 assert langJava -> stdenv.lib.versionOlder version "7";
 
+# Note [Windows Exception Handling]
+# sjlj (short jump long jump) exception handling makes no sense on x86_64,
+# it's forcably slowing programs down as it produces a constant overhead.
+# On x86_64 we have SEH (Structured Exception Handling) and we should use
+# that. On i686, we do not have SEH, and have to use sjlj with dwarf2.
+# Hence it's now conditional on x86_32 (i686 is 32bit).
+#
+# ref: https://stackoverflow.com/questions/15670169/what-is-difference-between-sjlj-vs-dwarf-vs-seh
+
+
 let
   inherit (stdenv)
     buildPlatform hostPlatform targetPlatform
@@ -55,8 +65,11 @@ let
       "--with-gnu-as"
       "--with-gnu-ld"
       "--disable-debug"
-      "--enable-sjlj-exceptions"
       "--disable-win32-registry"
+    ] ++ lib.optionals (crossMingw && targetPlatform.isx86_32) [
+      # See Note [Windows Exception Handling]
+      "--enable-sjlj-exceptions"
+      "--with-dwarf2"
     ] else [
       (if crossDarwin then "--with-sysroot=${lib.getLib libcCross}/share/sysroot"
        else                "--with-headers=${lib.getDev libcCross}${libcCross.incdir or "/include"}")
@@ -78,11 +91,9 @@ let
       # musl at least, disable: https://git.buildroot.net/buildroot/commit/?id=873d4019f7fb00f6a80592224236b3ba7d657865
       "--disable-libmpx"
     ] ++ lib.optionals crossMingw [
-      "--enable-sjlj-exceptions"
       "--enable-hash-synchronization"
       "--enable-libssp"
       "--disable-nls"
-      "--with-dwarf2"
       # To keep ABI compatibility with upstream mingw-w64
       "--enable-fully-dynamic-string"
     ] ++ lib.optional (targetPlatform.libc == "newlib") "--with-newlib"