Fix bug: scheme_to_interface must clear the floating-point register

stack.  Otherwise, inexplicable floating-point errors can occur.  An
example of such an error can be created by typing `-1e-194' at a
freshly-booted scheme.

The actual problem is somewhat involved.  The C compiler assumes that
all of the floating-point registers are empty on the entry to any
procedure.  As a consequence of this assumption, it feels free to use
all 8 of the registers.  However, if this assumption is violated, a C
procedure that uses too many registers will signal a floating-point
stack overflow.

However, the Scheme compiler does not follow this convention; more
precisely, since it was originally designed to use reasonable
floating-point hardware, in which the floating-point registers are
organized as a bank of registers rather than a stack, it doesn't do
anything about popping registers when it is done with them.  As a
consequence, floating-point operations generated by the Scheme
compiler often leave values on the floating-point stack.

To work around this without redesigning the compiler, the
compiled-code interface clears out the stack whenever it transfers
between C and Scheme code.  The bug fixed by this revision was a
transfer point that did not have code to clear out the stack.

diff --git a/v7/src/microcode/cmpauxmd/i386.m4 b/v7/src/microcode/cmpauxmd/i386.m4
index 4400c65f0248a3b7f56d3092867c1d09b164ea52..4b64e58e7ce7f5a0b71acdcd885d678c91a9ff66 100644 (file)
--- a/v7/src/microcode/cmpauxmd/i386.m4
+++ b/v7/src/microcode/cmpauxmd/i386.m4
@@ -1,6 +1,6 @@
 ### -*-Midas-*-
 ###
-###    $Id: i386.m4,v 1.46 1997/05/01 01:26:16 cph Exp $
+###    $Id: i386.m4,v 1.47 1997/05/05 06:59:35 cph Exp $
 ###
 ###    Copyright (c) 1992-97 Massachusetts Institute of Technology
 ###
@@ -566,6 +566,19 @@ define_debugging_label(scheme_to_interface_call)
 \f
 define_hook_label(scheme_to_interface)
 define_debugging_label(scheme_to_interface)
+IF387(`
+       OP(cmp,l)       TW(IMM(0),ABS(EVR(i387_presence)))
+       je      scheme_to_interface_proceed
+       ffree   ST(0)                                   # Free floating "regs"
+       ffree   ST(1)
+       ffree   ST(2)
+       ffree   ST(3)
+       ffree   ST(4)
+       ffree   ST(5)
+       ffree   ST(6)
+       ffree   ST(7)
+scheme_to_interface_proceed:
+')
 ifdef(`HACK_SEGMENT_REGS',
 `      OP(push,l)      LOF(36,regs)                    # 4th utility arg
        OP(push,l)      REG(eax)                        # Save utility index