Optimize merging of scalar loads for 32-byte vectors [X86, AVX]

Fix the poor codegen seen in PR21710 ( http://llvm.org/bugs/show_bug.cgi?id=21710 ).
Before we crack 32-byte build vectors into smaller chunks (and then subsequently
glue them back together), we should look for the easy case where we can just load
all elements in a single op.

An example of the codegen change is:

From:

vmovss  16(%rdi), %xmm1
vmovups (%rdi), %xmm0
vinsertps       $16, 20(%rdi), %xmm1, %xmm1
vinsertps       $32, 24(%rdi), %xmm1, %xmm1
vinsertps       $48, 28(%rdi), %xmm1, %xmm1
vinsertf128     $1, %xmm1, %ymm0, %ymm0
retq

To:

vmovups (%rdi), %ymm0
retq

Differential Revision: http://reviews.llvm.org/D6536



git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@223518 91177308-0d34-0410-b5e6-96231b3b80d8

lib/Target/X86/X86ISelLowering.cpp

@@ -6995,13 +6995,18 @@ X86TargetLowering::LowerBUILD_VECTOR(SDValue Op, SelectionDAG &DAG) const {
   if (IsAllConstants)
     return SDValue();
 
-  // For AVX-length vectors, build the individual 128-bit pieces and use
+  // For AVX-length vectors, see if we can use a vector load to get all of the
+  // elements, otherwise build the individual 128-bit pieces and use
   // shuffles to put them in place.
   if (VT.is256BitVector() || VT.is512BitVector()) {
     SmallVector<SDValue, 64> V;
     for (unsigned i = 0; i != NumElems; ++i)
       V.push_back(Op.getOperand(i));
 
+    // Check for a build vector of consecutive loads.
+    if (SDValue LD = EltsFromConsecutiveLoads(VT, V, dl, DAG, false))
+      return LD;
+
     EVT HVT = EVT::getVectorVT(*DAG.getContext(), ExtVT, NumElems/2);
 
     // Build both the lower and upper subvector.

test/CodeGen/X86/vec_loadsingles.ll

@@ -1,12 +1,99 @@
-; RUN: llc < %s -march=x86 -mattr=+sse2 | grep movq
-
-define <4 x float> @a(<4 x float> %a, float* nocapture %p) nounwind readonly {
-entry:
-	%tmp1 = load float* %p
-	%vecins = insertelement <4 x float> undef, float %tmp1, i32 0
-	%add.ptr = getelementptr float* %p, i32 1
-	%tmp5 = load float* %add.ptr
-	%vecins7 = insertelement <4 x float> %vecins, float %tmp5, i32 1
-	ret <4 x float> %vecins7
+; RUN: llc < %s -mtriple=x86_64-unknown-unknown -mattr=+avx,-slow-unaligned-mem-32 | FileCheck %s --check-prefix=ALL --check-prefix=FAST32
+; RUN: llc < %s -mtriple=x86_64-unknown-unknown -mattr=+avx,+slow-unaligned-mem-32 | FileCheck %s --check-prefix=ALL --check-prefix=SLOW32
+
+define <4 x float> @merge_2_floats(float* nocapture %p) nounwind readonly {
+  %tmp1 = load float* %p
+  %vecins = insertelement <4 x float> undef, float %tmp1, i32 0
+  %add.ptr = getelementptr float* %p, i32 1
+  %tmp5 = load float* %add.ptr
+  %vecins7 = insertelement <4 x float> %vecins, float %tmp5, i32 1
+  ret <4 x float> %vecins7
+
+; ALL-LABEL: merge_2_floats
+; ALL: vmovq
+; ALL-NEXT: retq
+}
+
+define <4 x float> @merge_4_floats(float* %ptr) {
+  %a = load float* %ptr, align 8
+  %vec = insertelement <4 x float> undef, float %a, i32 0
+  %idx1 = getelementptr inbounds float* %ptr, i64 1
+  %b = load float* %idx1, align 8
+  %vec2 = insertelement <4 x float> %vec, float %b, i32 1
+  %idx3 = getelementptr inbounds float* %ptr, i64 2
+  %c = load float* %idx3, align 8
+  %vec4 = insertelement <4 x float> %vec2, float %c, i32 2
+  %idx5 = getelementptr inbounds float* %ptr, i64 3
+  %d = load float* %idx5, align 8
+  %vec6 = insertelement <4 x float> %vec4, float %d, i32 3
+  ret <4 x float> %vec6
+
+; ALL-LABEL: merge_4_floats
+; ALL: vmovups
+; ALL-NEXT: retq
+}
+
+; PR21710 ( http://llvm.org/bugs/show_bug.cgi?id=21710 ) 
+; Make sure that 32-byte vectors are handled efficiently.
+; If the target has slow 32-byte accesses, we should still generate
+; 16-byte loads.
+
+define <8 x float> @merge_8_floats(float* %ptr) {
+  %a = load float* %ptr, align 4
+  %vec = insertelement <8 x float> undef, float %a, i32 0
+  %idx1 = getelementptr inbounds float* %ptr, i64 1
+  %b = load float* %idx1, align 4
+  %vec2 = insertelement <8 x float> %vec, float %b, i32 1
+  %idx3 = getelementptr inbounds float* %ptr, i64 2
+  %c = load float* %idx3, align 4
+  %vec4 = insertelement <8 x float> %vec2, float %c, i32 2
+  %idx5 = getelementptr inbounds float* %ptr, i64 3
+  %d = load float* %idx5, align 4
+  %vec6 = insertelement <8 x float> %vec4, float %d, i32 3
+  %idx7 = getelementptr inbounds float* %ptr, i64 4
+  %e = load float* %idx7, align 4
+  %vec8 = insertelement <8 x float> %vec6, float %e, i32 4
+  %idx9 = getelementptr inbounds float* %ptr, i64 5
+  %f = load float* %idx9, align 4
+  %vec10 = insertelement <8 x float> %vec8, float %f, i32 5
+  %idx11 = getelementptr inbounds float* %ptr, i64 6
+  %g = load float* %idx11, align 4
+  %vec12 = insertelement <8 x float> %vec10, float %g, i32 6
+  %idx13 = getelementptr inbounds float* %ptr, i64 7
+  %h = load float* %idx13, align 4
+  %vec14 = insertelement <8 x float> %vec12, float %h, i32 7
+  ret <8 x float> %vec14
+
+; ALL-LABEL: merge_8_floats
+
+; FAST32: vmovups
+; FAST32-NEXT: retq
+
+; SLOW32: vmovups
+; SLOW32: vinsertf128
+; SLOW32-NEXT: retq
+}
+
+define <4 x double> @merge_4_doubles(double* %ptr) {
+  %a = load double* %ptr, align 8
+  %vec = insertelement <4 x double> undef, double %a, i32 0
+  %idx1 = getelementptr inbounds double* %ptr, i64 1
+  %b = load double* %idx1, align 8
+  %vec2 = insertelement <4 x double> %vec, double %b, i32 1
+  %idx3 = getelementptr inbounds double* %ptr, i64 2
+  %c = load double* %idx3, align 8
+  %vec4 = insertelement <4 x double> %vec2, double %c, i32 2
+  %idx5 = getelementptr inbounds double* %ptr, i64 3
+  %d = load double* %idx5, align 8
+  %vec6 = insertelement <4 x double> %vec4, double %d, i32 3
+  ret <4 x double> %vec6
+
+; ALL-LABEL: merge_4_doubles
+; FAST32: vmovups
+; FAST32-NEXT: retq
+
+; SLOW32: vmovups
+; SLOW32: vinsertf128
+; SLOW32-NEXT: retq
 }