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Re: mips-tdep.c: Sign-extend pointers for n32
On Wed, 19 Dec 2007, Daniel Jacobowitz wrote:
> I happen to know otherwise. I once wasted several days tracking down
> a bug in the Linux kernel's IP checksumming implementation which
> resulted in incorrectly extended values in registers; there are MIPS
> parts which really do behave unpredictably when you use 32-bit
> arithmetic operations on them (specifically the Broadcom SB1).
I recall the issue and the problem was broken inline assembly, which used
64-bit operations to produce results associated with 32-bit integer types
and register constraints. This is a different issue and is doomed to fail
indeed.
> And I believe a compiler would be justified in using such instructions
> on a signed char argument.
Yes, of course -- it is actually meant to and thus will never produce
results outside the sign-extended 32-bit range (remember that before
extending, bits 31:16 or 31:8 as appropriate are zeroes, so after a
zero-extension the values still fall within the signed 32-bit range).
And most ALU operations do not care about the signedness, but there are
two exceptions that came to my mind after I sent you the previous letter,
specifically signed "div/mult", and GCC indeed uses signed/unsigned load
operations for 16/8-bit types as appropriate, so even though chasing the
relevant document that would state it explicitly seems a non-trivial task,
I think sign-extension of such small signed integers is implied.
Also n32 builds on compatibility with o32 and the most definite o32
document which is: "SYSTEM V APPLICATION BINARY INTERFACE, MIPS RISC
Processor Supplement, 3rd Edition" says:
"All integer-valued arguments are passed as 32-bit words, with signed or
unsigned bytes and halfwords expanded (promoted) as necessary."
Thus I have taken your suggestion into account and modified the change
further as follows and regression tested it as previously, successfully.
Well, I guess it actually means we do not really have a terribly good
coverage here -- I suppose a set of test cases that would check that
promotion is performed correctly with manual calls would be useful.
2007-12-19 David Ung <davidu@mips.com>
Maciej W. Rozycki <macro@mips.com>
* mips-tdep.c (mips_n32n64_push_dummy_call): Sign-extend
integers and 32-bit pointers as required by the ABI.
OK to apply?
Maciej
12741-0.diff
Index: binutils-quilt/src/gdb/mips-tdep.c
===================================================================
--- binutils-quilt.orig/src/gdb/mips-tdep.c 2007-12-19 15:15:45.000000000 +0000
+++ binutils-quilt/src/gdb/mips-tdep.c 2007-12-20 16:13:31.000000000 +0000
@@ -3184,8 +3184,21 @@
purpose register. */
if (argreg <= MIPS_LAST_ARG_REGNUM)
{
- LONGEST regval =
- extract_unsigned_integer (val, partial_len);
+ LONGEST regval;
+
+ /* Sign extend pointers, 32-bit integers and signed
+ 16-bit and 8-bit integers; everything else is taken
+ as is. */
+
+ if ((partial_len == 4
+ && (typecode == TYPE_CODE_PTR
+ || typecode == TYPE_CODE_INT))
+ || (partial_len < 4
+ && typecode == TYPE_CODE_INT
+ && !TYPE_UNSIGNED (arg_type)))
+ regval = extract_signed_integer (val, partial_len);
+ else
+ regval = extract_unsigned_integer (val, partial_len);
/* A non-floating-point argument being passed in a
general register. If a struct or union, and if