[llvm][SPIRV] Expose fast popcnt support for SPIR-V targets (#109845)

This adds the TTI predicate for conveying the availability of fast
`popcnt`, which subsequently allows passes like `LoopIdiomRecognize` to
collapse known popcount patterns. Since SPIR-V natively exposes
`OpBitcount`, it seems preferable to compress the resulting code, and
retain the information, even if a concrete target might have to expand
back into a loop structure.

Author: AlexVlx

llvm/lib/Target/SPIRV/SPIRVTargetTransformInfo.h

@@ -24,6 +24,7 @@
 namespace llvm {
 class SPIRVTTIImpl : public BasicTTIImplBase<SPIRVTTIImpl> {
   using BaseT = BasicTTIImplBase<SPIRVTTIImpl>;
+  using TTI = TargetTransformInfo;
 
   friend BaseT;
 
@@ -37,6 +38,16 @@ class SPIRVTTIImpl : public BasicTTIImplBase<SPIRVTTIImpl> {
   explicit SPIRVTTIImpl(const SPIRVTargetMachine *TM, const Function &F)
       : BaseT(TM, F.getDataLayout()), ST(TM->getSubtargetImpl(F)),
         TLI(ST->getTargetLowering()) {}
+
+  TTI::PopcntSupportKind getPopcntSupport(unsigned TyWidth) {
+    // SPIR-V natively supports OpBitcount, per 3.53.14 in the spec, as such it
+    // is reasonable to assume the Op is fast / preferable to the expanded loop.
+    // Furthermore, this prevents information being lost if transforms are
+    // applied to SPIR-V before lowering to a concrete target.
+    if (!isPowerOf2_32(TyWidth) || TyWidth > 64)
+      return TTI::PSK_Software; // Arbitrary bit-width INT is not core SPIR-V.
+    return TTI::PSK_FastHardware;
+  }
 };
 
 } // namespace llvm

llvm/test/Transforms/LoopIdiom/SPIRV/lit.local.cfg

@@ -0,0 +1,2 @@
+if not "SPIRV" in config.root.targets:
+    config.unsupported = True

llvm/test/Transforms/LoopIdiom/SPIRV/popcnt.ll

@@ -0,0 +1,105 @@
+; RUN: opt -passes=loop-idiom -mtriple=spirv32-- -S < %s | FileCheck %s
+; RUN: opt -passes=loop-idiom -mtriple=spirv64-- -S < %s | FileCheck %s
+
+; Mostly copied from x86 version.
+
+;To recognize this pattern:
+;int popcount(unsigned long long a) {
+;    int c = 0;
+;    while (a) {
+;        c++;
+;        a &= a - 1;
+;    }
+;    return c;
+;}
+;
+
+; CHECK-LABEL: @popcount_i64
+; CHECK: entry
+; CHECK: llvm.ctpop.i64
+; CHECK: ret
+define i32 @popcount_i64(i64 %a) nounwind uwtable readnone ssp {
+entry:
+  %tobool3 = icmp eq i64 %a, 0
+  br i1 %tobool3, label %while.end, label %while.body
+
+while.body:                                       ; preds = %entry, %while.body
+  %c.05 = phi i32 [ %inc, %while.body ], [ 0, %entry ]
+  %a.addr.04 = phi i64 [ %and, %while.body ], [ %a, %entry ]
+  %inc = add nsw i32 %c.05, 1
+  %sub = add i64 %a.addr.04, -1
+  %and = and i64 %sub, %a.addr.04
+  %tobool = icmp eq i64 %and, 0
+  br i1 %tobool, label %while.end, label %while.body
+
+while.end:                                        ; preds = %while.body, %entry
+  %c.0.lcssa = phi i32 [ 0, %entry ], [ %inc, %while.body ]
+  ret i32 %c.0.lcssa
+}
+
+; CHECK-LABEL: @popcount_i32
+; CHECK: entry
+; CHECK: llvm.ctpop.i32
+; CHECK: ret
+define i32 @popcount_i32(i32 %a) nounwind uwtable readnone ssp {
+entry:
+  %tobool3 = icmp eq i32 %a, 0
+  br i1 %tobool3, label %while.end, label %while.body
+
+while.body:                                       ; preds = %entry, %while.body
+  %c.05 = phi i32 [ %inc, %while.body ], [ 0, %entry ]
+  %a.addr.04 = phi i32 [ %and, %while.body ], [ %a, %entry ]
+  %inc = add nsw i32 %c.05, 1
+  %sub = add i32 %a.addr.04, -1
+  %and = and i32 %sub, %a.addr.04
+  %tobool = icmp eq i32 %and, 0
+  br i1 %tobool, label %while.end, label %while.body
+
+while.end:                                        ; preds = %while.body, %entry
+  %c.0.lcssa = phi i32 [ 0, %entry ], [ %inc, %while.body ]
+  ret i32 %c.0.lcssa
+}
+
+; To recognize this pattern:
+;int popcount(unsigned long long a, int mydata1, int mydata2) {
+;    int c = 0;
+;    while (a) {
+;        c++;
+;        a &= a - 1;
+;        mydata1 *= c;
+;        mydata2 *= (int)a;
+;    }
+;    return c + mydata1 + mydata2;
+;}
+
+; CHECK-LABEL: @popcount2
+; CHECK: entry
+; CHECK: llvm.ctpop.i64
+; CHECK: ret
+define i32 @popcount2(i64 %a, i32 %mydata1, i32 %mydata2) nounwind uwtable readnone ssp {
+entry:
+  %tobool9 = icmp eq i64 %a, 0
+  br i1 %tobool9, label %while.end, label %while.body
+
+while.body:                                       ; preds = %entry, %while.body
+  %c.013 = phi i32 [ %inc, %while.body ], [ 0, %entry ]
+  %mydata2.addr.012 = phi i32 [ %mul1, %while.body ], [ %mydata2, %entry ]
+  %mydata1.addr.011 = phi i32 [ %mul, %while.body ], [ %mydata1, %entry ]
+  %a.addr.010 = phi i64 [ %and, %while.body ], [ %a, %entry ]
+  %inc = add nsw i32 %c.013, 1
+  %sub = add i64 %a.addr.010, -1
+  %and = and i64 %sub, %a.addr.010
+  %mul = mul nsw i32 %inc, %mydata1.addr.011
+  %conv = trunc i64 %and to i32
+  %mul1 = mul nsw i32 %conv, %mydata2.addr.012
+  %tobool = icmp eq i64 %and, 0
+  br i1 %tobool, label %while.end, label %while.body
+
+while.end:                                        ; preds = %while.body, %entry
+  %c.0.lcssa = phi i32 [ 0, %entry ], [ %inc, %while.body ]
+  %mydata2.addr.0.lcssa = phi i32 [ %mydata2, %entry ], [ %mul1, %while.body ]
+  %mydata1.addr.0.lcssa = phi i32 [ %mydata1, %entry ], [ %mul, %while.body ]
+  %add = add i32 %mydata2.addr.0.lcssa, %mydata1.addr.0.lcssa
+  %add2 = add i32 %add, %c.0.lcssa
+  ret i32 %add2
+}