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Use the fucomi[p] instructions to perform floating point comparisons instead

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of the fucom[p][p] instructions.  This allows us to code generate this function

bool %test(double %X, double %Y) {
        %C = setlt double %Y, %X
        ret bool %C
}

... into:

test:
        fld QWORD PTR [%ESP + 4]
        fld QWORD PTR [%ESP + 12]
        fucomip %ST(1)
        fstp %ST(0)
        setb %AL
        movsx %EAX, %AL
        ret

where before we generated:

test:
        fld QWORD PTR [%ESP + 4]
        fld QWORD PTR [%ESP + 12]
        fucompp
**      fnstsw
**      sahf
        setb %AL
        movsx %EAX, %AL
        ret

The two marked instructions (which are the ones eliminated) are very bad,
because they serialize execution of the processor.  These instructions are
available on the PPRO and later, but since we already use cmov's we aren't
losing any portability.

I retained the old code for the day when we decide we want to support back
to the 386.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@12852 91177308-0d34-0410-b5e6-96231b3b80d8
This commit is contained in:
Chris Lattner
2004-04-12 01:43:36 +00:00
parent c040bca4b9
commit 8d2822e7f1
2 changed files with 14 additions and 6 deletions
Download Patch File Download Diff File Expand all files Collapse all files
lib/Target/X86/InstSelectSimple.cpp
+7 -3
View File
@@ -929,9 +929,13 @@ unsigned ISel::EmitComparison(unsigned OpNum, Value *Op0, Value *Op1,
BuildMI(*MBB, IP, X86::CMP32rr, 2).addReg(Op0r).addReg(Op1r);
break;
case cFP:
BuildMI(*MBB, IP, X86::FpUCOM, 2).addReg(Op0r).addReg(Op1r);
BuildMI(*MBB, IP, X86::FNSTSW8r, 0);
BuildMI(*MBB, IP, X86::SAHF, 1);
if (0) { // for processors prior to the P6
BuildMI(*MBB, IP, X86::FpUCOM, 2).addReg(Op0r).addReg(Op1r);
BuildMI(*MBB, IP, X86::FNSTSW8r, 0);
BuildMI(*MBB, IP, X86::SAHF, 1);
} else {
BuildMI(*MBB, IP, X86::FUCOMIr, 2).addReg(Op0r).addReg(Op1r);
}
break;
case cLong:
lib/Target/X86/X86ISelSimple.cpp
+7 -3
View File
@@ -929,9 +929,13 @@ unsigned ISel::EmitComparison(unsigned OpNum, Value *Op0, Value *Op1,
BuildMI(*MBB, IP, X86::CMP32rr, 2).addReg(Op0r).addReg(Op1r);
break;
case cFP:
BuildMI(*MBB, IP, X86::FpUCOM, 2).addReg(Op0r).addReg(Op1r);
BuildMI(*MBB, IP, X86::FNSTSW8r, 0);
BuildMI(*MBB, IP, X86::SAHF, 1);
if (0) { // for processors prior to the P6
BuildMI(*MBB, IP, X86::FpUCOM, 2).addReg(Op0r).addReg(Op1r);
BuildMI(*MBB, IP, X86::FNSTSW8r, 0);
BuildMI(*MBB, IP, X86::SAHF, 1);
} else {
BuildMI(*MBB, IP, X86::FUCOMIr, 2).addReg(Op0r).addReg(Op1r);
}
break;
case cLong: