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--- md.h 2009-10-01 20:46:43.000000000 +0200
+++ md.h 2014-12-16 22:52:34.000000000 +0100
@@ -161,6 +161,19 @@
#define MD_JB_SP 34
#elif defined(__amd64__)
#define MD_JB_SP 2
+/*
+ * The following comment is taken from src/lib/libc/amd64/gen/makecontext.c
+ * It explains why we specifically break the alignment to 64 byte boundary
+ * that is unconditionally enforced by sched.c, by setting MD_STACK_PAD_SIZE
+ * to this funky value.
+ * Without this, anything using va_start() & friends fails unpredictably.
+ */
+ /*
+ * Account for arguments on stack and do the funky C entry alignment.
+ * This means that we need an 8-byte-odd alignment since the ABI expects
+ * the return address to be pushed, thus breaking the 16 byte alignment.
+ */
+#define MD_STACK_PAD_SIZE 8
#else
#error Unknown CPU architecture
#endif
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