Fix Windows stack allocations.

On Windows we use the function `win32AllocStack` to do stack
allocations in 4k blocks and insert a stack check afterwards
to ensure the allocation returned a valid block.

The problem is this function does something that by C semantics
is pointless. The stack allocated value can never escape the
function, and the stack isn't used so the compiler just optimizes
away the entire function body.

After considering a bunch of other possibilities I think the simplest
fix is to just disable optimizations for the function.

Alternatively inline assembly is an option but the stack check function
doesn't have a very portable name as it relies on e.g. `libgcc`.

Thanks to Sergey Vinokurov for helping diagnose and test.

Test Plan: ./validate

Reviewers: bgamari, erikd, simonmar

Reviewed By: bgamari

Subscribers: rwbarton, thomie, carter

GHC Trac Issues: #14669

Differential Revision: https://phabricator.haskell.org/D4343

1 files changed, 10 insertions, 0 deletions

diff --git a/includes/Stg.h b/includes/Stg.h
index f377e50d98..2e023470ad 100644
--- a/includes/Stg.h
+++ b/includes/Stg.h
@@ -204,6 +204,16 @@
 
 #define STG_UNUSED    GNUC3_ATTRIBUTE(__unused__)
 
+/* Prevent functions from being optimized.
+   See Note [Windows Stack allocations] */
+#if defined(__clang__)
+#define STG_NO_OPTIMIZE __attribute__((optnone))
+#elif defined(__GNUC__) || defined(__GNUG__)
+#define STG_NO_OPTIMIZE __attribute__((optimize("O0")))
+#else
+#define STG_NO_OPTIMIZE /* nothing */
+#endif
+
 /* -----------------------------------------------------------------------------
    Global type definitions
    -------------------------------------------------------------------------- */