[AArch64] MachineCombiner msub matching

[vfdff]

Pattern should be sorted in priority order since the pattern evalutor stops checking as soon as it finds a faster sequence. so for a * b - c * d, we prefer to match the 2nd operands of sub is a multiplication, then we can use msub to fold them.

Refer to https://www.slideshare.net/chimerawang/instruction-combine-in-llvm

Fix #84152

[llvmbot]

@llvm/pr-subscribers-backend-aarch64

Author: Allen (vfdff)

Changes

Pattern should be sorted in priority order since the pattern evalutor stops checking as soon as it finds a faster sequence. so for a * b - c * d, we prefer to match the 2nd operands of sub, which can be use msub to fold them.

Refer to https://www.slideshare.net/chimerawang/instruction-combine-in-llvm

Fix #84152

---

Full diff: https://github.com/llvm/llvm-project/pull/84267.diff

2 Files Affected:

• (modified) llvm/lib/Target/AArch64/AArch64InstrInfo.cpp (+1-1)
• (added) llvm/test/CodeGen/AArch64/scalar-mla-mls.ll (+31)

diff --git a/llvm/lib/Target/AArch64/AArch64InstrInfo.cpp b/llvm/lib/Target/AArch64/AArch64InstrInfo.cpp
index 5df691f35275df..5893f76dbd5544 100644
--- a/llvm/lib/Target/AArch64/AArch64InstrInfo.cpp
+++ b/llvm/lib/Target/AArch64/AArch64InstrInfo.cpp
@@ -6110,8 +6110,8 @@ static bool getMaddPatterns(MachineInstr &Root,
     setFound(AArch64::MADDXrrr, 2, AArch64::XZR, MCP::MULADDX_OP2);
     break;
   case AArch64::SUBWrr:
-    setFound(AArch64::MADDWrrr, 1, AArch64::WZR, MCP::MULSUBW_OP1);
     setFound(AArch64::MADDWrrr, 2, AArch64::WZR, MCP::MULSUBW_OP2);
+    setFound(AArch64::MADDWrrr, 1, AArch64::WZR, MCP::MULSUBW_OP1);
     break;
   case AArch64::SUBXrr:
     setFound(AArch64::MADDXrrr, 1, AArch64::XZR, MCP::MULSUBX_OP1);
diff --git a/llvm/test/CodeGen/AArch64/scalar-mla-mls.ll b/llvm/test/CodeGen/AArch64/scalar-mla-mls.ll
new file mode 100644
index 00000000000000..36ac36701fa8aa
--- /dev/null
+++ b/llvm/test/CodeGen/AArch64/scalar-mla-mls.ll
@@ -0,0 +1,31 @@
+; NOTE: Assertions have been autogenerated by utils/update_llc_test_checks.py
+; RUN: llc < %s -verify-machineinstrs -mtriple=aarch64-none-linux-gnu -mattr=+neon | FileCheck %s
+
+define ptr @test_scalar_msub(ptr %a, ptr %b) {
+; CHECK-LABEL: test_scalar_msub:
+; CHECK:       // %bb.0: // %entry
+; CHECK-NEXT:    ldp w8, w11, [x1]
+; CHECK-NEXT:    ldp w9, w10, [x0]
+; CHECK-NEXT:    mul w12, w8, w9
+; CHECK-NEXT:    mul w8, w10, w8
+; CHECK-NEXT:    madd w8, w11, w9, w8
+; CHECK-NEXT:    msub w9, w11, w10, w12
+; CHECK-NEXT:    stp w9, w8, [x0]
+; CHECK-NEXT:    ret
+entry:
+  %0 = load i32, ptr %a, align 4
+  %1 = load i32, ptr %b, align 4
+  %mul = mul nsw i32 %1, %0
+  %_M_imag = getelementptr inbounds i8, ptr %a, i64 4
+  %2 = load i32, ptr %_M_imag, align 4
+  %_M_imag.i = getelementptr inbounds i8, ptr %b, i64 4
+  %3 = load i32, ptr %_M_imag.i, align 4
+  %mul3 = mul nsw i32 %3, %2
+  %sub = sub nsw i32 %mul, %mul3
+  %mul6 = mul nsw i32 %3, %0
+  %mul9 = mul nsw i32 %2, %1
+  %add = add nsw i32 %mul6, %mul9
+  store i32 %add, ptr %_M_imag, align 4
+  store i32 %sub, ptr %a, align 4
+  ret ptr %a
+}

[davemgreen]

Would the same thing be needed here too for 64bit?

[vfdff]

yes, I'll fix that, it has similar issue, thanks https://gcc.godbolt.org/z/79GnMPnY1

[vfdff]

Done, add msub matching for i64, thanks

[davemgreen]

LGTM, thanks