From e407dbe9fff3bfae06d00d4d5e38d5a8fb88c402 Mon Sep 17 00:00:00 2001
From: Chris Lattner <sabre@nondot.org>
Date: Mon, 12 Apr 2004 01:43:36 +0000
Subject: [PATCH] Use the fucomi[p] instructions to perform floating point
 comparisons instead 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.

llvm-svn: 12852
---
 llvm/lib/Target/X86/InstSelectSimple.cpp | 10 +++++++---
 1 file changed, 7 insertions(+), 3 deletions(-)

diff --git a/llvm/lib/Target/X86/InstSelectSimple.cpp b/llvm/lib/Target/X86/InstSelectSimple.cpp
index a2e7e6661b73..c8802ed74c17 100644
--- a/llvm/lib/Target/X86/InstSelectSimple.cpp
+++ b/llvm/lib/Target/X86/InstSelectSimple.cpp
@@ -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: