IRgen: Add a -fuse-register-sized-bitfield-access option, for testing.

 - Changes bit-field access policy to try to use (aligned) register sized accesses.

The idea here is that by using larger accesses we expose more coalescing
potential to the backend when we have situations like adjacent bit-fields in the
same structure (which is common), and that the backend should be smart enough to
narrow the accesses down when no coalescing is done or when it is shown not to
be profitable.

--
$ clang -m32 -O3 -S -o - t.c
_f0:                                    ## @f0
	pushl	%ebp
	movl	%esp, %ebp
	movl	8(%ebp), %eax
	movb	(%eax), %cl
	andb	$-128, %cl
	orb	$1, %cl
	movb	%cl, (%eax)
	movb	1(%eax), %cl
	andb	$-128, %cl
	orb	$1, %cl
	movb	%cl, 1(%eax)
	movb	2(%eax), %cl
	andb	$-128, %cl
	orb	$1, %cl
	movb	%cl, 2(%eax)
	movb	3(%eax), %cl
	andb	$-128, %cl
	orb	$1, %cl
	movb	%cl, 3(%eax)
	popl	%ebp
	ret

$ clang -m32 -O3 -S -o - t.c -Xclang -fuse-register-sized-bitfield-access
_f0:                                    ## @f0
	pushl	%ebp
	movl	%esp, %ebp
	movl	8(%ebp), %eax
	movl	$-2139062144, %ecx      ## imm = 0xFFFFFFFF80808080
	andl	(%eax), %ecx
	orl	$16843009, %ecx         ## imm = 0x1010101
	movl	%ecx, (%eax)
	popl	%ebp
	ret
--

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@133532 91177308-0d34-0410-b5e6-96231b3b80d8

2 files changed, 17 insertions, 0 deletions

diff --git a/lib/CodeGen/CGRecordLayoutBuilder.cpp b/lib/CodeGen/CGRecordLayoutBuilder.cpp
index 0d72f854ba..6daf2ea371 100644
--- a/lib/CodeGen/CGRecordLayoutBuilder.cpp
+++ b/lib/CodeGen/CGRecordLayoutBuilder.cpp
@@ -18,6 +18,7 @@
 #include "clang/AST/DeclCXX.h"
 #include "clang/AST/Expr.h"
 #include "clang/AST/RecordLayout.h"
+#include "clang/Frontend/CodeGenOptions.h"
 #include "CodeGenTypes.h"
 #include "CGCXXABI.h"
 #include "llvm/DerivedTypes.h"
@@ -268,6 +269,18 @@ CGBitFieldInfo CGBitFieldInfo::MakeInfo(CodeGenTypes &Types,
   unsigned AccessedTargetBits = 0;       // The number of target bits accessed.
   unsigned AccessWidth = TypeSizeInBits; // The current access width to attempt.
 
+  // If requested, widen the initial bit-field access to be register sized. The
+  // theory is that this is most likely to allow multiple accesses into the same
+  // structure to be coalesced, and that the backend should be smart enough to
+  // narrow the store if no coalescing is ever done.
+  //
+  // The subsequent code will handle align these access to common boundaries and
+  // guaranteeing that we do not access past the end of the structure.
+  if (Types.getCodeGenOpts().UseRegisterSizedBitfieldAccess) {
+    if (AccessWidth < Types.getTarget().getRegisterWidth())
+      AccessWidth = Types.getTarget().getRegisterWidth();
+  }
+
   // Round down from the field offset to find the first access position that is
   // at an aligned offset of the initial access type.
   uint64_t AccessStart = FieldOffset - (FieldOffset % AccessWidth);
diff --git a/lib/Frontend/CompilerInvocation.cpp b/lib/Frontend/CompilerInvocation.cpp
index 977f351e42..61887e8eb2 100644
--- a/lib/Frontend/CompilerInvocation.cpp
+++ b/lib/Frontend/CompilerInvocation.cpp
@@ -133,6 +133,8 @@ static void CodeGenOptsToArgs(const CodeGenOptions &Opts,
     Res.push_back("-fno-common");
   if (Opts.ForbidGuardVariables)
     Res.push_back("-fforbid-guard-variables");
+  if (Opts.UseRegisterSizedBitfieldAccess)
+    Res.push_back("-fuse-register-sized-bitfield-access");
   if (Opts.NoImplicitFloat)
     Res.push_back("-no-implicit-float");
   if (Opts.OmitLeafFramePointer)
@@ -956,6 +958,8 @@ static void ParseCodeGenArgs(CodeGenOptions &Opts, ArgList &Args, InputKind IK,
   Opts.DisableLLVMOpts = Args.hasArg(OPT_disable_llvm_optzns);
   Opts.DisableRedZone = Args.hasArg(OPT_disable_red_zone);
   Opts.ForbidGuardVariables = Args.hasArg(OPT_fforbid_guard_variables);
+  Opts.UseRegisterSizedBitfieldAccess = Args.hasArg(
+    OPT_fuse_register_sized_bitfield_access);
   Opts.RelaxedAliasing = Args.hasArg(OPT_relaxed_aliasing);
   Opts.DwarfDebugFlags = Args.getLastArgValue(OPT_dwarf_debug_flags);
   Opts.MergeAllConstants = !Args.hasArg(OPT_fno_merge_all_constants);