[RISCV] Rework IDiv and FDiv pipes on SiFive7

[michaelmaitland]

Set BufferSize=0 and remove Super pipes for these resources.

[llvmbot]

@llvm/pr-subscribers-backend-risc-v

Author: Michael Maitland (michaelmaitland)

Changes

Set BufferSize=0 and remove Super pipes for these resources.

---

Full diff: https://github.com/llvm/llvm-project/pull/73970.diff

1 Files Affected:

• (modified) llvm/lib/Target/RISCV/RISCVSchedSiFive7.td (+2-2)

diff --git a/llvm/lib/Target/RISCV/RISCVSchedSiFive7.td b/llvm/lib/Target/RISCV/RISCVSchedSiFive7.td
index 53ef9d1baf7b59a..261c22ea35317e2 100644
--- a/llvm/lib/Target/RISCV/RISCVSchedSiFive7.td
+++ b/llvm/lib/Target/RISCV/RISCVSchedSiFive7.td
@@ -213,12 +213,12 @@ let SchedModel = SiFive7Model in {
 let BufferSize = 0 in {
 def SiFive7PipeA       : ProcResource<1>;
 def SiFive7PipeB       : ProcResource<1>;
+def SiFive7IDiv        : ProcResource<1> { let Super = SiFive7PipeB; } // Int Division
+def SiFive7FDiv        : ProcResource<1> { let Super = SiFive7PipeB; } // FP Division/Sqrt
 def SiFive7PipeV       : ProcResource<1>;
 }
 
 let BufferSize = 1 in {
-def SiFive7IDiv        : ProcResource<1> { let Super = SiFive7PipeB; } // Int Division
-def SiFive7FDiv        : ProcResource<1> { let Super = SiFive7PipeB; } // FP Division/Sqrt
 def SiFive7VA          : ProcResource<1> { let Super = SiFive7PipeV; } // Arithmetic sequencer
 def SiFive7VL          : ProcResource<1> { let Super = SiFive7PipeV; } // Load sequencer
 def SiFive7VS          : ProcResource<1> { let Super = SiFive7PipeV; } // Store sequencer

[topperc]

I don't know why these need a Super class. Their usages already say SiFive7PipeB in the ResourceCycles explicitly.

[wangpc-pp]

IIUC, the bahavior is different. If there is no Super, SiFive7PipeB will be released after 1 cycle; if Super is specified, SiFive7PipeB will still be occupied after 1 cycle as SiFive7IDiv implictly use it. I don't know the details about the real pipeline, is it desired?

[michaelmaitland]

I didn't mean to keep the Super class. As @wangpc-pp points out that the description of the PR says it is removed. I will push a fixup.

[wangpc-pp]

The PR description says Super is removed but we still have it?

[wangpc-pp]

IIUC, the bahavior is different. If there is no Super, SiFive7PipeB will be released after 1 cycle; if Super is specified, SiFive7PipeB will still be occupied after 1 cycle as SiFive7IDiv implictly use it. I don't know the details about the real pipeline, is it desired?

[wangpc-pp]

LGTM.
(Is this not covered by some llvm-mca tests?)

[michaelmaitland]

(Is this not covered by some llvm-mca tests?)

I just added a test case. Check out the commit that adds it to see the diff (I precomitted the test)