Align stack to 16 byte boundary in x86-64's C_to_interface.

Otherwise, parts of the microcode fail spectacularly when compiled
with `gcc -O3'.

diff --git a/src/microcode/cmpauxmd/x86-64.m4 b/src/microcode/cmpauxmd/x86-64.m4
index fbf7417e38afa5a027915537a4953fe8c8cbc529..5560acfc80ed9bad3934b1eb4fbcd697de4276e7 100644 (file)
--- a/src/microcode/cmpauxmd/x86-64.m4
+++ b/src/microcode/cmpauxmd/x86-64.m4
@@ -532,6 +532,13 @@ x86_64_initialize_no_fp:
        leave
        ret
 
+# Note:  The AMD64 ABI mandates that on entry to a function, RSP - 8
+# must be a multiple of 0x10; that is, the stack must be 128-bit
+# aligned.  We push six quadwords onto the stack, but there is already
+# a return address on the stack, for a total of seven quadwords, which
+# is misaligned.  Hence we push an extra dummy zero onto the stack,
+# which we must pop off in interface_to_C.
+
 define_c_label(C_to_interface)
        OP(push,q)      REG(rbp)                        # Link according
        OP(mov,q)       TW(REG(rsp),REG(rbp))           #  to C's conventions
@@ -540,6 +547,7 @@ define_c_label(C_to_interface)
        OP(push,q)      REG(r13)
        OP(push,q)      REG(r14)
        OP(push,q)      REG(r15)
+       OP(push,q)      IMM(0)                          # Align stack
        OP(mov,q)       TW(REG(rdi),REG(rdx))           # Entry point
                                                        # Preserve frame ptr
        OP(mov,q)       TW(REG(rbp),ABS(EVR(C_Frame_Pointer)))
@@ -670,6 +678,10 @@ IF387(`
 interface_to_C_proceed:')
 
        OP(mov,q)       TW(REG(rdx),REG(rax))           # Set up result
+       # We need a dummy register for the POP (which is three bytes
+       # shorter than ADD $8,RSP); since we're about to pop into r15
+       # anyway, we may as well use that.
+       OP(pop,q)       REG(r15)                        # Undo stack alignment
        OP(pop,q)       REG(r15)                        # Restore callee-saves
        OP(pop,q)       REG(r14)                        #  registers
        OP(pop,q)       REG(r13)