[X86] Add some initial add i64 test coverage for #142308 (#166929)

Pulled from the abandoned #144066 patch

Author: RKSimon

llvm/test/CodeGen/X86/narrow-add-i64.ll

@@ -0,0 +1,94 @@
+; NOTE: Assertions have been autogenerated by utils/update_llc_test_checks.py
+; RUN: llc < %s -mtriple=i686-unknown-unknown | FileCheck %s --check-prefix=X86
+; RUN: llc < %s -mtriple=x86_64-unknown-unknown | FileCheck %s --check-prefix=X64
+
+define i64 @test_add_i64_i16_const(i16 %a) nounwind {
+; X86-LABEL: test_add_i64_i16_const:
+; X86:       # %bb.0:
+; X86-NEXT:    movzwl {{[0-9]+}}(%esp), %eax
+; X86-NEXT:    addl $42, %eax
+; X86-NEXT:    xorl %edx, %edx
+; X86-NEXT:    retl
+;
+; X64-LABEL: test_add_i64_i16_const:
+; X64:       # %bb.0:
+; X64-NEXT:    movzwl %di, %eax
+; X64-NEXT:    addq $42, %rax
+; X64-NEXT:    retq
+  %zext_a = zext i16 %a to i64
+  %sum = add nuw nsw i64 %zext_a, 42
+  ret i64 %sum
+}
+
+; TODO: First 48 bits are all zeros so we can safely truncate to 32 bit additon
+define i64 @test_add_i64_i16_zext(i16 %a, i16 %b) nounwind {
+; X86-LABEL: test_add_i64_i16_zext:
+; X86:       # %bb.0:
+; X86-NEXT:    movzwl {{[0-9]+}}(%esp), %ecx
+; X86-NEXT:    movzwl {{[0-9]+}}(%esp), %eax
+; X86-NEXT:    addl %ecx, %eax
+; X86-NEXT:    xorl %edx, %edx
+; X86-NEXT:    retl
+;
+; X64-LABEL: test_add_i64_i16_zext:
+; X64:       # %bb.0:
+; X64-NEXT:    movzwl %di, %ecx
+; X64-NEXT:    movzwl %si, %eax
+; X64-NEXT:    addq %rcx, %rax
+; X64-NEXT:    retq
+  %zext_a = zext i16 %a to i64
+  %zext_b = zext i16 %b to i64
+  %sum = add nuw nsw i64 %zext_a, %zext_b
+  ret i64 %sum
+}
+
+; Negative: Set the 32nd bit of a to force 64 bit addition, we do not truncate to 32 bit addition in this case
+define i64 @negative_test_add_i64_i16(i16 %a) nounwind {
+; X86-LABEL: negative_test_add_i64_i16:
+; X86:       # %bb.0:
+; X86-NEXT:    movzwl {{[0-9]+}}(%esp), %eax
+; X86-NEXT:    addl $42, %eax
+; X86-NEXT:    movl $1, %edx
+; X86-NEXT:    retl
+;
+; X64-LABEL: negative_test_add_i64_i16:
+; X64:       # %bb.0:
+; X64-NEXT:    movzwl %di, %ecx
+; X64-NEXT:    movabsq $4294967338, %rax # imm = 0x10000002A
+; X64-NEXT:    addq %rcx, %rax
+; X64-NEXT:    retq
+  %zext_a = zext i16 %a to i64
+  %or_a = or i64 %zext_a, 4294967296
+  %sum = add nuw nsw i64 %or_a, 42
+  ret i64 %sum
+}
+
+; Negative: We don't truncate to 32 bit addition in case of sign extension
+define i64 @negative_test_add_i64_i16_sext(i16 %a, i16 %b) nounwind {
+; X86-LABEL: negative_test_add_i64_i16_sext:
+; X86:       # %bb.0:
+; X86-NEXT:    pushl %esi
+; X86-NEXT:    movswl {{[0-9]+}}(%esp), %ecx
+; X86-NEXT:    movl %ecx, %esi
+; X86-NEXT:    sarl $31, %esi
+; X86-NEXT:    movswl {{[0-9]+}}(%esp), %eax
+; X86-NEXT:    movl %eax, %edx
+; X86-NEXT:    sarl $31, %edx
+; X86-NEXT:    addl %ecx, %eax
+; X86-NEXT:    adcl %esi, %edx
+; X86-NEXT:    popl %esi
+; X86-NEXT:    retl
+;
+; X64-LABEL: negative_test_add_i64_i16_sext:
+; X64:       # %bb.0:
+; X64-NEXT:    # kill: def $esi killed $esi def $rsi
+; X64-NEXT:    # kill: def $edi killed $edi def $rdi
+; X64-NEXT:    movswq %di, %rcx
+; X64-NEXT:    movswq %si, %rax
+; X64-NEXT:    addq %rcx, %rax
+; X64-NEXT:    retq
+  %sext_a = sext i16 %a to i64
+  %sext_b = sext i16 %b to i64
+  %sum = add nuw nsw i64 %sext_a, %sext_b
+  ret i64 %sum
+}