[AArch64] Report icmp as free if it can be folded into ands

[AZero13]

Since changing the backend to fold x >= 1 / x < 1 -> x > 0 / x <= 0 and x <= -1 / x > -1 -> x > 0 / x <= 0, this should be reflected in the cost.

[llvmbot]

@llvm/pr-subscribers-llvm-analysis

@llvm/pr-subscribers-backend-aarch64

Author: AZero13 (AZero13)

Changes

Since changing the backend to fold x >= 1 / x < 1 -> x > 0 / x <= 0 and x <= -1 / x > -1 -> x > 0 / x <= 0, this should be reflected in the cost.

---

Full diff: https://github.com/llvm/llvm-project/pull/143286.diff

1 Files Affected:

• (modified) llvm/lib/Target/AArch64/AArch64TargetTransformInfo.cpp (+15-6)

diff --git a/llvm/lib/Target/AArch64/AArch64TargetTransformInfo.cpp b/llvm/lib/Target/AArch64/AArch64TargetTransformInfo.cpp
index 68aec80f07e1d..72b60eaa45b78 100644
--- a/llvm/lib/Target/AArch64/AArch64TargetTransformInfo.cpp
+++ b/llvm/lib/Target/AArch64/AArch64TargetTransformInfo.cpp
@@ -4346,14 +4346,23 @@ InstructionCost AArch64TTIImpl::getCmpSelInstrCost(
   }
 
   // Treat the icmp in icmp(and, 0) as free, as we can make use of ands.
-  // FIXME: This can apply to more conditions and add/sub if it can be shown to
-  // be profitable.
   if (ValTy->isIntegerTy() && ISD == ISD::SETCC && I &&
-      ICmpInst::isEquality(VecPred) &&
+      !CmpInst::isUnsigned(VecPred) &&
       TLI->isTypeLegal(TLI->getValueType(DL, ValTy)) &&
-      match(I->getOperand(1), m_Zero()) &&
-      match(I->getOperand(0), m_And(m_Value(), m_Value())))
-    return 0;
+      match(I->getOperand(0), m_And(m_Value(), m_Value()))) {
+    if (match(I->getOperand(1), m_Zero()))
+      return 0;
+
+    // x >= 1 / x < 1 -> x > 0 / x <= 0
+    if (match(I->getOperand(1), m_One()) &&
+        (VecPred == CmpInst::ICMP_SLT || VecPred == CmpInst::ICMP_SGE))
+      return 0;
+
+    // x <= -1 / x > -1 -> x > 0 / x <= 0
+    if (match(I->getOperand(1), m_AllOnes()) &&
+        (VecPred == CmpInst::ICMP_SLE || VecPred == CmpInst::ICMP_SGT))
+      return 0;
+  }
 
   // The base case handles scalable vectors fine for now, since it treats the
   // cost as 1 * legalization cost.

[AZero13]

@davemgreen Thoughts?

[davemgreen]

Hi - I think this sounds OK, some of the cases will be canonicalized by the mid-end to compare against 0. Out of interest do you have some motivating example you are trying to optimize, or is this a general improvement as you were looking at the backend patterns?

Can you add some extra tests for each of the cases?

[AZero13]

The cost model needs to be accurate as possible

[SamTebbs33]

LGTM, thanks. Do let @davemgreen have another look though.

[davemgreen]

Sure, LGTM. Are you happy for this to be merged?

[AZero13]

Sure, LGTM. Are you happy for this to be merged?

Yes