[InstCombine] Known-bits optimization for ARM MVE VADC.

[simon_tatham]

Summary

The MVE VADC instruction reads and writes the carry bit at bit 29 of
the FPSCR register. The corresponding ACLE intrinsic is specified to
work with an integer in which the carry bit is stored at bit 0. So if
a user writes a code sequence in C that passes the carry from one VADC
to the next, like this,

s0 = vadcq_u32(a0, b0, &carry);
s1 = vadcq_u32(a1, b1, &carry);

then clang will generate IR for each of those operations that shifts
the carry bit up into bit 29 before the VADC, and after it, shifts it
back down and masks off all but the low bit. But in this situation
what you really wanted was two consecutive VADC instructions, so that
the second one directly reads the value left in FPSCR by the first,
without wasting several instructions on pointlessly clearing the other
flag bits in between.

This commit explains to InstCombine that the other bits of the flags
operand don't matter, and adds a test that demonstrates that all the
code between the two VADC instructions can be optimized away as a
result.

[dmgreen]

Sounds good.

We may also be able to do something similar in ISel, during the optimisation there, but this looks like a more general fix (and I'm not sure that would add a lot over what is already here).

llvm/lib/Transforms/InstCombine/InstCombineCalls.cpp

3299

This looks like something else entirely!

It's setting the range metadata on vminv's?

3334

Is this ever not 32? If so the 32 below should be the same value

3338

I think the last parameter is depth, and the default is 0? If so it might as well be left off.

llvm/test/CodeGen/Thumb2/mve-intrinsics/vadc-multiple.ll

20

Can you include a test for arm_mve_vadc_predicated?

[simon_tatham]

llvm/lib/Transforms/InstCombine/InstCombineCalls.cpp

3299

D'oh! Well spotted. That's what I get for writing my commit messages at the last minute.

[simon_tatham]

llvm/lib/Transforms/InstCombine/InstCombineCalls.cpp

3334

It should always be 32, because that intrinsic argument represents a value you can VMSR into FPSCR. I'll get rid of CarryWidth completely and make it an assertion.

[simon_tatham]

Moved the vmin range metadata into a separate patch.

[simon_tatham]

Rebased to current master.

[dmgreen]

LGTM. Thanks

[simon_tatham]

(Rebased to current master; no change)

llvm/lib/Transforms/InstCombine/InstCombineCalls.cpp

case Intrinsic::aarch64_crypto_aese: {
     if (match(KeyArg, m_ZeroInt()) &&
         match(DataArg, m_Xor(m_Value(Data), m_Value(Key)))) {
       II->setArgOperand(0, Data);
       II->setArgOperand(1, Key);
       return II;
     }
     break;
   }
+  case Intrinsic::arm_mve_minv_u:
+  case Intrinsic::arm_mve_minv_s: {
+    unsigned ScalarWidth = II->getArgOperand(1)
+                               ->getType()
+                               ->getVectorElementType()
+                               ->getScalarSizeInBits();
+
+    bool Modified = false;
+
+    KnownBits ScalarKnown(32);
+    if (SimplifyDemandedBits(II, 0, APInt::getLowBitsSet(32, ScalarWidth),
+                             ScalarKnown, 0))
+      Modified = true;
+    if (ScalarWidth < 32 && !II->getMetadata(LLVMContext::MD_range)) {
+      uint32_t Lo = 0, Hi = (uint32_t)1 << ScalarWidth;
+      if (IID == Intrinsic::arm_mve_minv_s) {
+        uint32_t Offset = Hi >> 1;
+        Lo -= Offset;
+        Hi -= Offset;
+      }
+      Type *IntTy32 = Type::getInt32Ty(II->getContext());
+      Metadata *M[] = {ConstantAsMetadata::get(ConstantInt::get(IntTy32, Lo)),
+                       ConstantAsMetadata::get(ConstantInt::get(IntTy32, Hi))};
+      II->setMetadata(LLVMContext::MD_range, MDNode::get(II->getContext(), M));
+      Modified = true;
+    }
+    if (Modified)
+      return II;
+    break;
+  }
+  case Intrinsic::arm_mve_vadc:
+  case Intrinsic::arm_mve_vadc_predicated: {
+    unsigned CarryOp =
+        (II->getIntrinsicID() == Intrinsic::arm_mve_vadc_predicated) ? 3 : 2;
+    Value *CarryArg = II->getArgOperand(CarryOp);
+    unsigned CarryWidth = CarryArg->getType()->getScalarSizeInBits();
+
+    KnownBits CarryKnown(32);
+    if (SimplifyDemandedBits(II, CarryOp, APInt::getOneBitSet(CarryWidth, 29),
+                             CarryKnown, 0))
+      return II;
+    break;
+  }
   case Intrinsic::amdgcn_rcp: {
     Value *Src = II->getArgOperand(0);
 
     // TODO: Move to ConstantFolding/InstSimplify?
     if (isa<UndefValue>(Src))
       return replaceInstUsesWith(CI, Src);
 
     if (const ConstantFP *C = dyn_cast<ConstantFP>(Src)) {

llvm/test/CodeGen/Thumb2/mve-intrinsics/vadc-multiple.ll

+; RUN: opt -instcombine -S %s | FileCheck --check-prefix=IR %s
+; RUN: opt -instcombine    %s | llc -mtriple=thumbv8.1m.main -mattr=+mve.fp -O3 -o - | FileCheck --check-prefix=ASM %s
+
+%struct.foo = type { [2 x <4 x i32>] }
+
+; Function Attrs: noinline nounwind optnone
+define arm_aapcs_vfpcc i32 @test_vadciq_multiple(%struct.foo %a, %struct.foo %b, i32 %carry) {
+entry:
+  %a.0 = extractvalue %struct.foo %a, 0, 0
+  %a.1 = extractvalue %struct.foo %a, 0, 1
+  %b.0 = extractvalue %struct.foo %b, 0, 0
+  %b.1 = extractvalue %struct.foo %b, 0, 1
+
+  %fpscr.in.0 = shl i32 %carry, 29
+  %outpair.0 = call { <4 x i32>, i32 } @llvm.arm.mve.vadc.v4i32(<4 x i32> %a.0, <4 x i32> %b.0, i32 %fpscr.in.0)
+  %fpscr.out.0 = extractvalue { <4 x i32>, i32 } %outpair.0, 1
+  %shifted.out.0 = lshr i32 %fpscr.out.0, 29
+  %carry.out.0 = and i32 1, %shifted.out.0
+  %fpscr.in.1 = shl i32 %carry.out.0, 29
+  %outpair.1 = call { <4 x i32>, i32 } @llvm.arm.mve.vadc.v4i32(<4 x i32> %a.1, <4 x i32> %b.1, i32 %fpscr.in.1)
+  %fpscr.out.1 = extractvalue { <4 x i32>, i32 } %outpair.1, 1
+  %shifted.out.1 = lshr i32 %fpscr.out.1, 29
+  %carry.out.1 = and i32 1, %shifted.out.1
+  ret i32 %carry.out.1
+}
+
+declare { <4 x i32>, i32 } @llvm.arm.mve.vadc.v4i32(<4 x i32>, <4 x i32>, i32)
+
+; Expect the transformation in between the two intrinsics, where the
+; fpscr-formatted output value is turned back into just the carry bit
+; at bit 0 and then back again for the next call, to be optimized away
+; completely in InstCombine, so that the FPSCR output from one
+; intrinsic is passed straight on to the next:
+
+; IR: %outpair.0 = call { <4 x i32>, i32 } @llvm.arm.mve.vadc.v4i32(<4 x i32> %a.0, <4 x i32> %b.0, i32 %fpscr.in.0)
+; IR: %fpscr.out.0 = extractvalue { <4 x i32>, i32 } %outpair.0, 1
+; IR: %outpair.1 = call { <4 x i32>, i32 } @llvm.arm.mve.vadc.v4i32(<4 x i32> %a.1, <4 x i32> %b.1, i32 %fpscr.out.0)
+
+; And this is the assembly language we expect at the end of it, with
+; the two vadc.i32 instructions right next to each other, and the
+; second one implicitly reusing the FPSCR written by the first.
+
+; ASM: test_vadciq_multiple:
+; ASM:      lsls r0, r0, #29
+; ASM-NEXT: vmsr fpscr_nzcvqc, r0
+; ASM-NEXT: vadc.i32 q0, q0, q2
+; ASM-NEXT: vadc.i32 q0, q1, q3
+; ASM-NEXT: vmrs r0, fpscr_nzcvqc
+; ASM-NEXT: ubfx r0, r0, #29, #1
+; ASM-NEXT: bx lr