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We can wait on either VM, EXP or LGKM. The waits are independent. Without this patch, a wait inserted because of one of them would also wait for all the previous others. This patch makes s_wait only wait for the ones we need for the next instruction. Here's an example of subtle perf reduction this patch solves: This is without the patch: buffer_load_format_xyzw v[8:11], v0, s[44:47], 0 idxen buffer_load_format_xyzw v[12:15], v0, s[48:51], 0 idxen s_load_dwordx4 s[44:47], s[8:9], 0xc s_waitcnt lgkmcnt(0) buffer_load_format_xyzw v[16:19], v0, s[52:55], 0 idxen s_load_dwordx4 s[48:51], s[8:9], 0x10 s_waitcnt vmcnt(1) buffer_load_format_xyzw v[20:23], v0, s[44:47], 0 idxen The s_waitcnt vmcnt(1) is useless. The reason it is added is because the last buffer_load_format_xyzw needs s[44:47], which was issued by the first s_load_dwordx4. It waits for all VM before that call to have finished. Internally after every instruction, 3 counters (for VM, EXP and LGTM) are updated after every instruction. For example buffer_load_format_xyzw will increase the VM counter, and s_load_dwordx4 the LGKM one. Without the patch, for every defined register, the current 3 counters are stored, and are used to know how long to wait when an instruction needs the register. Because of that, the s[44:47] counter includes that to use the register you need to wait for the previous buffer_load_format_xyzw. Instead this patch stores only the counters that matter for the register, and puts zero for the other ones, since we don't need any wait for them. Patch by: Axel Davy Differential Revision: http://reviews.llvm.org/D11883 llvm-svn: 245755 |
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