[AArch64] Expand bcmp() for small block lengths

Patch D56593 by @courbet results in calls to `bcmp()` in some cases, should
the target support the it.  Unless `TTI::MemCmpExpansionOptions()`
is overridden by the target.

In a proprietary benchmark we see a performance drop of about 12% on PNG
compression before this patch, though it passes all tests.

This patch mirrors X86 for AArch64 and initializes
`TTI::MemCmpExpansionOptions()` to then expand calls to `bcmp()` when
appropriate.  No tuning of the parameters was performed, but, at this point,
it's enough to recover the performance drop above.

This problem also exists on ARM.  Once a consensus is reached for AArch64, we
can work to fix ARM as well.

Authors:
- Evandro Menezes (@evandro) <e.menezes@samsung.com>
- Brian Rzycki (@brzycki) <b.rzycki@samsung.com>

Differential revision: https://reviews.llvm.org/D64805

llvm-svn: 367898

1 files changed, 44 insertions, 0 deletions

diff --git a/llvm/test/CodeGen/AArch64/bcmp-inline-small.ll b/llvm/test/CodeGen/AArch64/bcmp-inline-small.ll
new file mode 100644
index 000000000000..da42b1d6863c
--- /dev/null
+++ b/llvm/test/CodeGen/AArch64/bcmp-inline-small.ll
@@ -0,0 +1,44 @@
+; RUN: llc -O2 < %s -mtriple=aarch64-linux-gnu                     | FileCheck %s --check-prefixes=CHECK,CHECKN
+; RUN: llc -O2 < %s -mtriple=aarch64-linux-gnu -mattr=strict-align | FileCheck %s --check-prefixes=CHECK,CHECKS
+
+declare i32 @bcmp(i8*, i8*, i64) nounwind readonly
+declare i32 @memcmp(i8*, i8*, i64) nounwind readonly
+
+define i1 @bcmp_b2(i8* %s1, i8* %s2) {
+entry:
+  %bcmp = call i32 @bcmp(i8* %s1, i8* %s2, i64 15)
+  %ret = icmp eq i32 %bcmp, 0
+  ret i1 %ret
+
+; CHECK-LABEL: bcmp_b2:
+; CHECK-NOT:   bl bcmp
+; CHECKN:      ldr  x
+; CHECKN-NEXT: ldr  x
+; CHECKN-NEXT: ldur x
+; CHECKN-NEXT: ldur x
+; CHECKS:      ldr  x
+; CHECKS-NEXT: ldr  x
+; CHECKS-NEXT: ldr  w
+; CHECKS-NEXT: ldr  w
+; CHECKS-NEXT: ldrh w
+; CHECKS-NEXT: ldrh w
+; CHECKS-NEXT: ldrb w
+; CHECKS-NEXT: ldrb w
+}
+
+define i1 @bcmp_bs(i8* %s1, i8* %s2) optsize {
+entry:
+  %memcmp = call i32 @memcmp(i8* %s1, i8* %s2, i64 31)
+  %ret = icmp eq i32 %memcmp, 0
+  ret i1 %ret
+
+; CHECK-LABEL: bcmp_bs:
+; CHECKN-NOT:  bl memcmp
+; CHECKN:      ldp  x
+; CHECKN-NEXT: ldp  x
+; CHECKN-NEXT: ldr  x
+; CHECKN-NEXT: ldr  x
+; CHECKN-NEXT: ldur x
+; CHECKN-NEXT: ldur x
+; CHECKS:      bl memcmp
+}