On Fri, Jul 3, 2015 at 3:10 PM, Vineet Gupta <vineet.gup...@synopsys.com> wrote:
> Hi,
>
> I have the following test case (reduced from Linux kernel sources) and it
> seems
> gcc is optimizing away the first loop iteration.
>
> arc-linux-gcc -c -O2 star-9000857057.c -fno-branch-count-reg --save-temps -mA7
>
> ----------->8-------------
> static inline int __test_bit(unsigned int nr, const volatile unsigned long
> *addr)
> {
> unsigned long mask;
>
> addr += nr >> 5;
> #if 0
> nr &= 0x1f;
> #endif
> mask = 1UL << nr;
> return ((mask & *addr) != 0);
> }
>
> int foo (int a, unsigned long *p)
> {
> int i;
> for (i = 63; i>=0; i--)
> {
> if (!(__test_bit(i, p)))
> continue;
> a += i;
> }
> return a;
> }
> ----------->8-------------
>
> gcc generates following
>
> ----------->8-------------
> .global foo
> .type foo, @function
> foo:
> ld_s r2,[r1,4] <---- dead code
> mov_s r2,63
> .align 4
> .L2:
> sub r2,r2,1 <-----SUB first
> cmp r2,-1
> jeq.d [blink]
> lsr r3,r2,5 <----- BUG: first @mask is (1 << 62) NOT (1 << 63)
> .align 2
> .L4:
> ld.as r3,[r1,r3]
> bbit0.nd r3,r2,@.L2
> add_s r0,r0,r2
> sub r2,r2,1
> cmp r2,-1
> bne.d @.L4
> lsr r3,r2,5
> j_s [blink]
> .size foo, .-foo
> .ident "GCC: (ARCv2 ISA Linux uClibc toolchain
> arc-2015.06-rc1-21-g21b2c4b83dfa)
> 4.8.4"
> ----------->8-------------
>
> For initial 32 loop operations, this test is effectively doing 64 bit
> operation,
> e.g. (1 << 63) in 32 bit regime. Is this supposed to be undefined, truncated
> to
> zero or port specific.
>
> If it is truncate to zero then generated code below is not correct as it
> needs to
> elide not just the first iteration (corresponding to i = 63) but 63..32
>
> Further ARCompact ISA provides that instructions involving bitpos operands
> BSET,
> BCLR, LSL can any number whatsoever, but core will only use the lower 5 bits
> (so
> clamping the bitpos to 0..31 w/o need for doing that in code.
>
> So is this a gcc bug, or some spec misinterpretation,.....
It is the C language standard that says that shifts like this invoke
undefined behavior.
Richard.
> TIA,
> -Vineet
