Pre-commit test cases for (sra (load)) -> (sextload) folds. NFC

[bjope]

Summary

Add test case to show missing folds for (sra (load)) -> (sextload).

[bjope]

Update comments.

[bjope]

Rebased (on top of D116929).

llvm/test/CodeGen/X86/combine-sra-load.ll

+; NOTE: Assertions have been autogenerated by utils/update_llc_test_checks.py
+; RUN: llc < %s -mtriple=x86_64-unknown-unknown | FileCheck %s --check-prefix=CHECK
+
+; FIXME: fold (sra (load i32), 16)) -> (sextload i16)
+define i32 @sra_half(i32* %p) {
+; CHECK-LABEL: sra_half:
+; CHECK:       # %bb.0:
+; CHECK-NEXT:    movl (%rdi), %eax
+; CHECK-NEXT:    sarl $16, %eax
+; CHECK-NEXT:    retq
+  %load = load i32, i32* %p
+  %shift = ashr i32 %load, 16
+  ret i32 %shift
+}
+
+; Vector version not folded.
+define <4 x i32> @sra_half_vec(<4 x i32>* %p) {
+; CHECK-LABEL: sra_half_vec:
+; CHECK:       # %bb.0:
+; CHECK-NEXT:    movdqa (%rdi), %xmm0
+; CHECK-NEXT:    psrad $16, %xmm0
+; CHECK-NEXT:    retq
+  %load = load <4 x i32>, <4 x i32>* %p
+  %shift = ashr <4 x i32> %load, <i32 16, i32 16, i32 16, i32 16>
+  ret <4 x i32> %shift
+}
+
+; FIXME: fold (sra (load i64), 48)) -> (sextload i16)
+define i64 @sra_large_shift(i64* %r) {
+; CHECK-LABEL: sra_large_shift:
+; CHECK:       # %bb.0:
+; CHECK-NEXT:    movq (%rdi), %rax
+; CHECK-NEXT:    sarq $48, %rax
+; CHECK-NEXT:    retq
+  %t0 = load i64, i64* %r
+  %conv = ashr i64 %t0, 48
+  ret i64 %conv
+}
+
+; Negative test, no fold expected.
+define i32 @sra_small_shift(i32* %p) {
+; CHECK-LABEL: sra_small_shift:
+; CHECK:       # %bb.0:
+; CHECK-NEXT:    movl (%rdi), %eax
+; CHECK-NEXT:    sarl $8, %eax
+; CHECK-NEXT:    retq
+  %load = load i32, i32* %p
+  %shift = ashr i32 %load, 8
+  ret i32 %shift
+}
+
+; This should be folded to a zextload.
+define i32 @sra_of_zextload(i16* %p) {
+; CHECK-LABEL: sra_of_zextload:
+; CHECK:       # %bb.0:
+; CHECK-NEXT:    movzbl 1(%rdi), %eax
+; CHECK-NEXT:    retq
+  %load = load i16, i16* %p
+  %zext = zext i16 %load to i32
+  %shift = ashr i32 %zext, 8
+  ret i32 %shift
+}
+
+; FIXME: fold (sra (sextload i16 to i32), 8) -> (sextload i8)
+define i32 @sra_of_sextload(i16* %p) {
+; CHECK-LABEL: sra_of_sextload:
+; CHECK:       # %bb.0:
+; CHECK-NEXT:    movswl (%rdi), %eax
+; CHECK-NEXT:    sarl $8, %eax
+; CHECK-NEXT:    retq
+  %load = load i16, i16* %p
+  %sext = sext i16 %load to i32
+  %shift = ashr i32 %sext, 8
+  ret i32 %shift
+}
+
+; Negative test. All bits loaded from memory are shifted out, so we can fold
+; away the shift.
+define i32 @sra_of_sextload_no_fold(i16* %p) {
+; CHECK-LABEL: sra_of_sextload_no_fold:
+; CHECK:       # %bb.0:
+; CHECK-NEXT:    movswl (%rdi), %eax
+; CHECK-NEXT:    sarl $16, %eax
+; CHECK-NEXT:    retq
+  %load = load i16, i16* %p
+  %sext = sext i16 %load to i32
+  %shift = ashr i32 %sext, 16
+  ret i32 %shift
+}
+
+; FIXME: Fold even if SRA has multiple uses.
+define i32 @sra_to_sextload_multiple_sra_uses(i32* %p) {
+; CHECK-LABEL: sra_to_sextload_multiple_sra_uses:
+; CHECK:       # %bb.0:
+; CHECK-NEXT:    movl (%rdi), %ecx
+; CHECK-NEXT:    sarl $16, %ecx
+; CHECK-NEXT:    movl %ecx, %eax
+; CHECK-NEXT:    xorl $6, %eax
+; CHECK-NEXT:    orl %ecx, %eax
+; CHECK-NEXT:    retq
+  %load = load i32, i32* %p
+  %shift = ashr i32 %load, 16
+  %use1 = xor i32 %shift, 6
+  %use2 = or i32 %shift, %use1
+  ret i32 %use2
+}