[PATCH] D13680: x86: preserve flags when folding atomic operations

[JF Bastien via llvm-commits]

jfb created this revision.
jfb added reviewers: rsmith, rtrieu.
jfb added subscribers: morisset, reames, llvm-commits.

D4796 taught LLVM to fold some atomic integer operations into a single
instruction. The pattern was unaware that the instructions clobbered
flags.

This patch adds the missing EFLAGS definition.

Floating point operations don't set flags, the subsequent fadd
optimization is therefore correct. The same applies for surrounding
load/store optimizations.

http://reviews.llvm.org/D13680

Files:
  lib/Target/X86/X86InstrCompiler.td
  test/CodeGen/X86/atomic-flags.ll

Index: test/CodeGen/X86/atomic-flags.ll
===================================================================
--- /dev/null
+++ test/CodeGen/X86/atomic-flags.ll
@@ -0,0 +1,38 @@
+; RUN: llc < %s -mtriple=x86_64-unknown-unknown -verify-machineinstrs | FileCheck %s
+; RUN: llc < %s -mtriple=i686-unknown-unknown -verify-machineinstrs | FileCheck %s
+
+; Make sure that flags are properly preserved despite atomic optimizations.
+
+define i32 @atomic_and_flags(i8* %p, i32 %a, i32 %b) {
+; CHECK-LABEL: atomic_and_flags:
+
+  ; Generate flags value, and use it.
+  ; CHECK:      cmpl
+  ; CHECK-NEXT: jne
+  %cmp = icmp eq i32 %a, %b
+  br i1 %cmp, label %L1, label %L2
+
+L1:
+  ; The following pattern will get folded.
+  ; CHECK: addb
+  %1 = load atomic i8, i8* %p seq_cst, align 1
+  %2 = add i8 %1, 2
+  store atomic i8 %2, i8* %p release, align 1
+
+  ; Use the comparison result again. We need to rematerialize the comparison
+  ; somehow. This test checks that cmpl gets emitted again, but any
+  ; rematerialization would work (the optimizer used to clobber the flags with
+  ; the add).
+  ; CHECK-NEXT: cmpl
+  ; CHECK-NEXT: jne
+  br i1 %cmp, label %L3, label %L4
+
+L2:
+  ret i32 2
+
+L3:
+  ret i32 3
+
+L4:
+  ret i32 4
+}
Index: lib/Target/X86/X86InstrCompiler.td
===================================================================
--- lib/Target/X86/X86InstrCompiler.td
+++ lib/Target/X86/X86InstrCompiler.td
@@ -774,12 +774,14 @@
         [(atomic_store_64 addr:$dst, (op
             (atomic_load_64 addr:$dst), GR64:$src))]>;
 }
-defm RELEASE_ADD : RELEASE_BINOP_MI<add>;
-defm RELEASE_AND : RELEASE_BINOP_MI<and>;
-defm RELEASE_OR  : RELEASE_BINOP_MI<or>;
-defm RELEASE_XOR : RELEASE_BINOP_MI<xor>;
-// Note: we don't deal with sub, because substractions of constants are
-// optimized into additions before this code can run
+let Defs = [EFLAGS] in {
+  defm RELEASE_ADD : RELEASE_BINOP_MI<add>;
+  defm RELEASE_AND : RELEASE_BINOP_MI<and>;
+  defm RELEASE_OR  : RELEASE_BINOP_MI<or>;
+  defm RELEASE_XOR : RELEASE_BINOP_MI<xor>;
+  // Note: we don't deal with sub, because substractions of constants are
+  //       optimized into additions before this code can run.
+}
 
 // Same as above, but for floating-point.
 // FIXME: imm version.


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