[llvm] [RISCV] Add a command line option to disable cost per use for compressed registers. (PR #83320)

[via llvm-commits]

llvmbot wrote:


<!--LLVM PR SUMMARY COMMENT-->

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

Author: Craig Topper (topperc)

<details>
<summary>Changes</summary>

I've seen cases where the cost per use increase the number of spills. Disabling improves the codegen for #<!-- -->79918.

I propose adding this option to allow easier experimentation.

---
Full diff: https://github.com/llvm/llvm-project/pull/83320.diff


1 Files Affected:

- (modified) llvm/lib/Target/RISCV/RISCVRegisterInfo.cpp (+6-1) 


``````````diff
diff --git a/llvm/lib/Target/RISCV/RISCVRegisterInfo.cpp b/llvm/lib/Target/RISCV/RISCVRegisterInfo.cpp
index 9d1f01dffaaf47..a68674b221d38e 100644
--- a/llvm/lib/Target/RISCV/RISCVRegisterInfo.cpp
+++ b/llvm/lib/Target/RISCV/RISCVRegisterInfo.cpp
@@ -30,6 +30,8 @@
 
 using namespace llvm;
 
+static cl::opt<bool> DisableCostPerUse("riscv-disable-cost-per-use",
+                                       cl::init(false), cl::Hidden);
 static cl::opt<bool>
     DisableRegAllocHints("riscv-disable-regalloc-hints", cl::Hidden,
                          cl::init(false),
@@ -712,7 +714,10 @@ void RISCVRegisterInfo::getOffsetOpcodes(const StackOffset &Offset,
 
 unsigned
 RISCVRegisterInfo::getRegisterCostTableIndex(const MachineFunction &MF) const {
-  return MF.getSubtarget<RISCVSubtarget>().hasStdExtCOrZca() ? 1 : 0;
+  return MF.getSubtarget<RISCVSubtarget>().hasStdExtCOrZca() &&
+                 !DisableCostPerUse
+             ? 1
+             : 0;
 }
 
 // Add two address hints to improve chances of being able to use a compressed

``````````

</details>


https://github.com/llvm/llvm-project/pull/83320