Implement function-sections for Haskell code, #8405

This adds a flag -split-sections that does similar things to
-split-objs, but using sections in single object files instead of
relying on the Satanic Splitter and other abominations. This is very
similar to the GCC flags -ffunction-sections and -fdata-sections.

The --gc-sections linker flag, which allows unused sections to actually
be removed, is added to all link commands (if the linker supports it) so
that space savings from having base compiled with sections can be
realized.

Supported both in LLVM and the native code-gen, in theory for all
architectures, but really tested on x86 only.

In the GHC build, a new SplitSections variable enables -split-sections
for relevant parts of the build.

Test Plan: validate with both settings of SplitSections

Reviewers: dterei, Phyx, austin, simonmar, thomie, bgamari

Reviewed By: simonmar, thomie, bgamari

Subscribers: hsyl20, erikd, kgardas, thomie

Differential Revision: https://phabricator.haskell.org/D1242

GHC Trac Issues: #8405

1 files changed, 26 insertions, 0 deletions

diff --git a/driver/utils/merge_sections.ld b/driver/utils/merge_sections.ld
new file mode 100644
index 0000000000..8c82ca09b5
--- /dev/null
+++ b/driver/utils/merge_sections.ld
@@ -0,0 +1,26 @@
+/* Linker script to undo -split-sections and merge all sections together when
+ * linking relocatable object files for GHCi.
+ * ld -r normally retains the individual sections, which is what you would want
+ * if the intention is to eventually link into a binary with --gc-sections, but
+ * it doesn't have a flag for directly doing what we want. */
+SECTIONS
+{
+    .text : {
+        *(.text*)
+    }
+    .rodata.cst16 : {
+        *(.rodata.cst16*)
+    }
+    .rodata : {
+        *(.rodata*)
+    }
+    .data.rel.ro : {
+        *(.data.rel.ro*)
+    }
+    .data : {
+        *(.data*)
+    }
+    .bss : {
+        *(.bss*)
+    }
+}