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This is an automatically generated e-mail. To reply, visit:
http://reviews.gem5.org/r/2119/
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Review request for Default.
Repository: gem5
Description
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mips: Floating point convert bug fix
Floating point convert instructions use the FloatConvertOp format defined
in src/arch/mips/isa/formats/fp.isa. The type of the operands in the ISA
description file (_sw for signed word, or _sf for signed float, etc.) is
used to create a type for the operand in C++. Then the operand is converted
using the fpConvert() function in src/arch/mips/utility.cc.
Here's what an fp cvt looks like in the generated execution function in
build/MIPS/arch/mips/generated/inorder_cpu_exec.cc::
uint32_t val = Fs;
Fd = fpConvert(WORD_TO_SINGLE, val);
The fpConvert function is in src/arch/mips/utility.cc::
uint64_t
fpConvert(ConvertType cvt_type, double fp_val)
{
switch (cvt_type)
{
case SINGLE_TO_DOUBLE:
{
double sdouble_val = fp_val;
void *sdouble_ptr = &sdouble_val;
uint64_t sdp_bits = *(uint64_t *) sdouble_ptr;
return sdp_bits;
}
case SINGLE_TO_WORD:
{
int32_t sword_val = (int32_t) fp_val;
void *sword_ptr = &sword_val;
uint64_t sword_bits= *(uint32_t *) sword_ptr;
return sword_bits;
}
(...)
}
}
Basically it does pointer casting to read values as a different type.
The bug is that _val_ is unsigned instead of signed. So if we are converting
from
a word to a float, and we want to convert 0xffffffff, we expect -1 to be passed
into
fpConvert(). Instead, we see MAX_INT passed in. Then fpConvert() converts _val_
to
MAX_INT in single-precision floating point, and we get the wrong value.
To fix it, I changed the signs of the convert operands from unsigned to signed
in the MIPS ISA description.
Then, I changed the FloatConvertOp format to insert a int32_t into the C++ code
instead
of a uint32_t.
Diffs
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src/arch/mips/isa/decoder.isa bdd606534bdc
src/arch/mips/isa/formats/fp.isa bdd606534bdc
Diff: http://reviews.gem5.org/r/2119/diff/
Testing
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I've tested this by writing a MIPS assembly test that converts a test set of
words to
floats. For example::
TEST( cvt.s.w, 0x4372a3d5, 0x4e86e548 )
TEST( cvt.s.w, 0xffffffff, 0xbf800000 )
.. where TEST is a test macro. The objdump looks like this for each test::
808000: 3c024372 lui v0,0x4372
808004: 3442a3d5 ori v0,v0,0xa3d5
808008: 46801120 cvt.s.w a0,v0
80800c: 241d000d li sp,13
808010: 3c014e86 lui at,0x4e86
808014: 3421e548 ori at,at,0xe548
808018: 14810095 bne a0,at,808270 <_fail>
Without this bug fix, all negative number tests fail.
Thanks,
Christopher Torng
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