When 32 bits are assigned to any of the standard registers, the upper 32 bits are implicitly set to zero. Intel is weird.

Didn't AMD invent the 64-bit extensions? -- Kim-Ee On Sun, Dec 9, 2012 at 3:07 PM, Axel Simon wrote:

On 09.12.2012, at 00:12, Yuras Shumovich wrote:

...

It looks wrong for me: the highest part of %rax remains uninitialized.

When 32 bits are assigned to any of the standard registers, the upper 32 bits are implicitly set to zero. Intel is weird.

Axel

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On Mon, 2012-12-10 at 10:58 +0000, Simon Marlow wrote:

On 08/12/12 23:12, Yuras Shumovich wrote:

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I tried to hack stg_putMVarzh directly:

if (enabled_capabilities == 1) { info = GET_INFO(mvar); } else { ("ptr" info) = ccall lockClosure(mvar "ptr"); }

You should use n_capabilities, not enabled_capabilities. The latter might be 1, even when there are multiple capabilities actually in use, while the RTS is in the process of migrating threads.

Could you please elaborate? setNumCapabilities is guarded with asquireAllCapabilities, so all threads are in scheduler. And threads will be migrated from disabled capabilities before they get a chance to put/take mvar. I changed my mind re enabled_capabilities/n_capabilities a number of times during the weekend. Most likely you are right, and I should use n_capabilities. But I'll appreciate if you find time to explain it for me.

...

But got no speedup at all. The generated asm (amd64):

movl $enabled_capabilities,%eax cmpq $1,%rax je .Lcgq .Lcgp: movq %rbx,%rdi subq $8,%rsp movl $0,%eax call lockClosure addq $8,%rsp .Lcgr: cmpq $stg_MVAR_CLEAN_info,%rax jne .Lcgu {...} .Lcgq: movq (%rbx),%rax jmp .Lcgr

It moves enabled_capabilities into %eax and then compares 1 with %rax. It looks wrong for me: the highest part of %rax remains uninitialized.

As Axel noted, this is correct.

The problem was that "movl $enabled_capabilities,%eax" loaded the address of enabled_capabilities, not a value. (Again, why does it use 32bit register? The value is 32bit on linux, but the address is 64bit, isn't it?) So the correct way to use C-land variable is: if (CInt[enabled_capabilities]) {...} Not very intuitive, but at least it works. Thanks, Yuras