llvm dev - Oct 2012 - [LLVMdev] Solicit code review (change to CodeGen)

Hi,

   The attached is the fix to radar://11663049. The optimization can be 
outlined by following rules:

     (select (x != c), e, c) -> select (x != c), e, x),
     (select (x == c), c, e) -> select (x == c), x, e)
where the <c> is an integer constant.

   The reason for this change is that : on x86, 
conditional-move-from-constant needs two instructions;
however, conditional-move-from-register need only one instruction.

    While the LowerSELECT() sounds to be the most convenient place for 
this optimization, it turns out to be a bad place.The reason is that by 
replacing the constant <c> with a symbolic value, it obscure some 
instruction-combining opportunities which would otherwise be very easy 
to spot. For that reason, I have to postpone the change to last 
instruction-combining phase.

    The change passes the test of "make check-all -C
<build-root/test"
and "make -C project/test-suite/SingleSource".

Thanks
Shuxin
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Index: lib/Target/X86/X86ISelLowering.cpp
==================================================================---
lib/Target/X86/X86ISelLowering.cpp	(revision 165638)
+++ lib/Target/X86/X86ISelLowering.cpp	(working copy)
@@ -14418,6 +14418,7 @@
       if (TrueC->getAPIntValue().ult(FalseC->getAPIntValue())) {
         CC = X86::GetOppositeBranchCondition(CC);
         std::swap(TrueC, FalseC);
+        std::swap(TrueOp, FalseOp);
       }
 
       // Optimize C ? 8 : 0 -> zext(setcc(C)) << 3.  Likewise for any
pow2/0.
@@ -14500,6 +14501,45 @@
       }
     }
   }
+
+  // Handle these cases:
+  //   (select (x != c), e, c) -> select (x != c), e, x),
+  //   (select (x == c), c, e) -> select (x == c), x, e) 
+  // where the c is an integer constant, and the "select" is the
combination
+  // of CMOV and CMP.
+  //
+  // The rationale for this change is that the conditional-move from a constant
+  // needs two instructions, however, conditional-move from a register needs 
+  // only one instruction.
+  // 
+  // CAVEAT: By replacing a constant with a symbolic value, it may obscure 
+  //  some instruction-combining opportunities. This opt needs to be 
+  //  postponed as late as possible.
+  //
+  if (!DCI.isBeforeLegalize() && !DCI.isBeforeLegalizeOps()) {
+    // the DCI.xxxx conditions are provided to postpone the optimization as 
+    // late as possible.
+
+    ConstantSDNode *CmpAgainst = 0;
+    if ((Cond.getOpcode() == X86ISD::CMP || Cond.getOpcode() == X86ISD::SUB)
&&
+        (CmpAgainst = dyn_cast<ConstantSDNode>(Cond.getOperand(1)))
&&
+        dyn_cast<ConstantSDNode>(Cond.getOperand(0)) == 0) {
+
+      if (CC == X86::COND_NE && 
+          CmpAgainst == dyn_cast<ConstantSDNode>(FalseOp)) {
+        CC = X86::GetOppositeBranchCondition(CC);
+        std::swap(TrueOp, FalseOp);
+      }
+
+      if (CC == X86::COND_E && 
+          CmpAgainst == dyn_cast<ConstantSDNode>(TrueOp)) {
+        SDValue Ops[] = { FalseOp, Cond.getOperand(0), N->getOperand(2),
Cond };
+        return DAG.getNode(X86ISD::CMOV, DL, N->getVTList (), Ops, 
+                           array_lengthof(Ops));
+      }
+    }
+  }
+
   return SDValue();
 }
 
Index: test/CodeGen/X86/select_const.ll
==================================================================---
test/CodeGen/X86/select_const.ll	(revision 0)
+++ test/CodeGen/X86/select_const.ll	(revision 0)
@@ -0,0 +1,16 @@
+; RUN: llc < %s -mtriple=x86_64-apple-darwin10 -mcpu=corei7 | FileCheck %s
+
+define i64 @test1(i64 %x) nounwind {
+entry:
+  %cmp = icmp eq i64 %x, 2
+  %add = add i64 %x, 1
+  %retval.0 = select i1 %cmp, i64 2, i64 %add
+  ret i64 %retval.0
+
+; CHECK: test1:
+; CHECK: leaq 1(%rdi), %rax
+; CHECK: cmpq $2, %rdi
+; CHECK: cmoveq %rdi, %rax
+; CHECK: ret
+
+}

LGTM. 

I will commit. 

On Oct 10, 2012, at 1:20 PM, Shuxin Yang <shuxin.llvm at gmail.com> wrote:
> Hi,
> 
>  The attached is the fix to radar://11663049. The optimization can be
outlined by following rules:
> 
>    (select (x != c), e, c) -> select (x != c), e, x),
>    (select (x == c), c, e) -> select (x == c), x, e)
> where the <c> is an integer constant.
> 
>  The reason for this change is that : on x86,
conditional-move-from-constant needs two instructions;
> however, conditional-move-from-register need only one instruction.
> 
>   While the LowerSELECT() sounds to be the most convenient place for this
optimization, it turns out to be a bad place.The reason is that by replacing the
constant <c> with a symbolic value, it obscure some instruction-combining
opportunities which would otherwise be very easy to spot. For that reason, I
have to postpone the change to last instruction-combining phase.
> 
>   The change passes the test of "make check-all -C
<build-root/test" and "make -C
project/test-suite/SingleSource".
> 
> Thanks
> Shuxin
> <diff.patch>_______________________________________________
> LLVM Developers mailing list
> LLVMdev at cs.uiuc.edu         http://llvm.cs.uiuc.edu
> http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev

Hi All,

I just started with the API of LLVM and have a beginner's question.

I'm trying to JIT compile the attached C program "getinmemory" to
an
inmemory module by a my attached example LLVM program.
After linking of this inmemory module against libc.so.6 I call the main 
routine of the C program.

Why do I have still malloc as a unresoveld externals ?  Contains the 
module "module_ex" not an fully executable program ?

Here are the printouts ( Linux):

------------------------
compile_to_module-return

clang -fdiagnostics-format=clang getinmemory.c -fsyntax-only -I 
/usr/include -I /usr/lib/gcc/i586-suse-linux/4.6/include -I 
/usr/src/linux/include -I /usr/src/linux/include/linux -v 
-fdollars-in-identifiers -fno-operator-names -ffp-contract=on 
-fobjc-runtime=macosx -triple i386-pc-linux-gnu
clang -cc1 version 3.2 based upon LLVM 3.2svn default target 
i386-pc-linux-gnu
#include "..." search starts here:
#include <...> search starts here:
  /usr/include
  /usr/lib/gcc/i586-suse-linux/4.6/include
  /usr/src/linux/include
  /usr/src/linux/include/linux
End of search list.
[ clip .. warnings]
Main
malloc
curl_global_init
curl_easy_init
curl_easy_setopt
WriteMemoryCallback
curl_easy_perform
curl_easy_cleanup
printf
free
curl_global_cleanup
main
realloc
exit
llvm.memcpy.p0i8.p0i8.i32
LLVM ERROR: Tried to execute an unknown external function: malloc
-----------------------

My target is to JIT compile e.g. the sources of a C/C++ library into 
executable inmemory code
in order to call the individual functions of the library. It's just a 
different kind of late bindings ...

Best Regards

Armin Steinhoff







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INCLUDES=-I /usr/local/include/clang -I/usr/local/include/llvm
-I/usr/local/include
LIBS= -L/home/CLING/build/Debug+Asserts/lib/clang/3.2/lib/linux
-lclang_rt.full-i386 -lclang_rt.profile-i386
-L/home/CLING/build/Debug+Asserts/lib \
-lclangFrontendTool -lclangFrontend -lclangDriver \
-lclangSerialization -lclangCodeGen -lclangParse -lclangSema \
-lclangStaticAnalyzerFrontend -lclangStaticAnalyzerCheckers \
-lclangStaticAnalyzerCore \
-lclangAnalysis -lclangARCMigrate -lclangRewrite \
-lclangEdit -lclangAST -lclangLex -lclangBasic \
-lLLVMAsmParser -lLLVMTableGen -lLLVMDebugInfo -lLLVMX86AsmParser
-lLLVMX86Disassembler -lLLVMX86CodeGen -lLLVMSelectionDAG -lLLVMAsmPrinter \
-lLLVMJIT -lLLVMMCDisassembler -lLLVMMCParser -lLLVMInstrumentation
-lLLVMInterpreter \
-lLLVMCodeGen -lLLVMipo -lLLVMVectorize -lLLVMScalarOpts -lLLVMInstCombine
-lLLVMLinker -lLLVMTransformUtils -lLLVMipa -lLLVMAnalysis -lLLVMArchive \
-lLLVMBitReader -lLLVMBitWriter -lLLVMMCJIT -lLLVMRuntimeDyld
-lLLVMExecutionEngine -lLLVMTarget -lLLVMMC -lLLVMObject -lLLVMCore
-lLLVMSupport \
-lLLVMX86Desc -lLLVMX86Info -lLLVMX86AsmPrinter -lLLVMX86Utils 

all : build_executable compile_to_module-return compile_to_module  


#build_executable :
#	g++ `llvm-config --cxxflags` $(INCLUDES)  -o build_executable
build_executable.cpp $(LIBS) -ldl -lpthread  #-Wl,-rpath-link
/home/CLING/build/Debug+Asserts/lib


#compile_to_module : 
#	g++ `llvm-config --cxxflags` $(INCLUDES)  -o compile_to_module
compile_to_module.cpp $(LIBS) -ldl -lpthread


compile_to_module-return :
	g++ `llvm-config --cxxflags` $(INCLUDES)  -o compile_to_module-return
compile_to_module-return.cpp $(LIBS) -ldl -lpthread