[PATCH 1/2]: FastFPE 'lfm' fix for denormalized numbers
FastFPE's implementation of the lfm opcode ('load floating-point
multiple') marks all floating-point numbers that it loads as normalized.
Unfortunately, zero and infinity are denormalized numbers, and FastFPE's
lfm will therefore corrupt any stored zero or infinity values reloaded
from memory.
The enclosed patch fixes this bug by testing for the two magic exponents
in FastFPE that indicate zero or infinity, and ensuring that the
'normalized' bit is cleared.
Minimal test cases that illustrate the problem are at
<http://www.booyaka.com/~paul/arm/fpa11_lfm_tests.tar.gz>. After patching
with the patch below, the result of the test cases is identical between
NWFPE and FastFPE. (The test cases are derived from code posted earlier
by Nicholas Clark <nick@unfortu.net> - thanks!)
Particular thanks go to Tony Lindgren <tony@atomide.com> for providing the
opportunity to catch the bug, and for rebuilding Linux kernels for his
Psion as required to test solutions.
The following bug reports in other software are known to be caused by this
bug:
. 'Re: [ID 20020504.002] 1 == 1 but 0 != 0'
<http://lists.debian.org/debian-arm/2002/debian-arm-200205/msg00004.html>
. 'Perl IPC::Open3 bug, affects apt-get, compiler issue?'
<http://lists.debian.org/debian-arm/2002/debian-arm-200206/msg00001.html>
- Paul
--- linux-2.4.19-rmk2/arch/arm/fastfpe/CPDT.S Thu Sep 26 12:09:16 2002
+++ linux-2.4.19-rmk2-pjw/arch/arm/fastfpe/CPDT.S Thu Sep 26 14:39:32 2002
@@ -402,16 +402,25 @@
.globl CPDT_lfm
CPDT_lfm:
add r2,r10,r0,lsr#8
- ldr r3,[r6],#4
- and r4,r3,#0x80000000
- orr r3,r3,#0x80000000
- str r4,[r2,#0]
- str r3,[r2,#4]
+ ldr r4,[r6],#4
+ and r3,r4,#0x80000000
+ str r3,[r2,#0]
ldr r3,[r6],#4
str r3,[r2,#8]
ldr r3,[r6],#4
str r3,[r2,#12]
+ cmp r3,#0x80000000 @ does the exp indicate zero?
+ biceq r4,r4,#0x80000000 @ if so, indicate 'denormalized'
+ beq CPDT_lfm_storer4
+ cmp r3,#0x7fffffff @ does the exp indicate inf or NaN?
+ biceq r4,r4,#0x80000000 @ if so, indicate 'denormalized'
+ beq CPDT_lfm_storer4
+ orrne r4,r4,#0x80000000 @ otherwise, set normalized bit
+
+CPDT_lfm_storer4:
+ str r4,[r2,#4]
+
add r0,r0,#1<<12
and r0,r0,#7<<12
subs r1,r1,#1
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