[AMDGPU] Mark S_NOP as having side effects #65745
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This prevents S_NOP from being rescheduled past other (side-effecting) instructions, which is useful because it is generally used to introduce a short delay or to avoid hazards. Currently this only affects MIR tests because the compiler itself only inserts nops in PostRAHazardRecognizer which runs after all scheduling.
jayfoad
commented
Sep 8, 2023
| ; CHECK-NEXT: {{ $}} | ||
| ; CHECK-NEXT: bb.1: | ||
| ; CHECK-NEXT: S_NOP 0, implicit %1.sub2 | ||
| ; CHECK-NEXT: S_NOP 0, implicit undef %4.sub0:vreg_128 |
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S_NOPs can't be reordered now, since they all have side effects.
jayfoad
commented
Sep 8, 2023
| ; GCN: bb.0: | ||
| ; GCN-NEXT: successors: %bb.1(0x80000000) | ||
| ; GCN-NEXT: {{ $}} | ||
| ; GCN-NEXT: S_NOP 0 |
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S_NOPs are no longer deleted by dead-mi-elimination.
jayfoad
commented
Sep 8, 2023
| ; GFX908-NEXT: ; implicit-def: $vgpr61 | ||
| ; GFX908-NEXT: s_nop 0 | ||
| ; GFX908-NEXT: s_nop 0 | ||
| ; GFX908-NEXT: s_nop 1 |
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GCNHazardRecognizer wants to delay for 2 cycles here. If there was already an S_NOP 0 here it would add another one, instead of modifying it to be an S_NOP 1.
arsenm
approved these changes
Sep 8, 2023
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This prevents S_NOP from being rescheduled past other (side-effecting)
instructions, which is useful because it is generally used to introduce
a short delay or to avoid hazards. Currently this only affects MIR tests
because the compiler itself only inserts nops in PostRAHazardRecognizer
which runs after all scheduling.