From 4e55044ff52029c2345639008aab5c7a1f21cfb8 Mon Sep 17 00:00:00 2001
From: Jakob Stoklund Olesen <stoklund@2pi.dk>
Date: Thu, 29 Mar 2012 17:22:39 +0000
Subject: [PATCH] Don't PRE compares.

CodeGenPrepare sinks compare instructions down to their uses to prevent
live flags and predicate registers across basic blocks.

PRE of a compare instruction prevents that, forcing the i1 compare
result into a general purpose register.  That is usually more expensive
than the redundant compare PRE was trying to eliminate in the first
place.

llvm-svn: 153657
---
 llvm/lib/Transforms/Scalar/GVN.cpp      |  9 +++-
 llvm/test/Transforms/GVN/pre-compare.ll | 68 +++++++++++++++++++++++++
 2 files changed, 76 insertions(+), 1 deletion(-)
 create mode 100644 llvm/test/Transforms/GVN/pre-compare.ll

diff --git a/llvm/lib/Transforms/Scalar/GVN.cpp b/llvm/lib/Transforms/Scalar/GVN.cpp
index 8b79d27b9521..38afe1b83d11 100644
--- a/llvm/lib/Transforms/Scalar/GVN.cpp
+++ b/llvm/lib/Transforms/Scalar/GVN.cpp
@@ -2328,7 +2328,14 @@ bool GVN::performPRE(Function &F) {
           CurInst->mayReadFromMemory() || CurInst->mayHaveSideEffects() ||
           isa<DbgInfoIntrinsic>(CurInst))
         continue;
-      
+
+      // Don't do PRE on compares. The PHI would prevent CodeGenPrepare from
+      // sinking the compare again, and it would force the code generator to
+      // move the i1 from processor flags or predicate registers into a general
+      // purpose register.
+      if (isa<CmpInst>(CurInst))
+        continue;
+
       // We don't currently value number ANY inline asm calls.
       if (CallInst *CallI = dyn_cast<CallInst>(CurInst))
         if (CallI->isInlineAsm())
diff --git a/llvm/test/Transforms/GVN/pre-compare.ll b/llvm/test/Transforms/GVN/pre-compare.ll
new file mode 100644
index 000000000000..18d0c2e1085c
--- /dev/null
+++ b/llvm/test/Transforms/GVN/pre-compare.ll
@@ -0,0 +1,68 @@
+; RUN: opt -gvn -S < %s | FileCheck %s
+
+; C source:
+;
+;   void f(int x) {
+;     if (x != 1)
+;       puts (x == 2 ? "a" : "b");
+;     for (;;) {
+;       puts("step 1");
+;       if (x == 2)
+;         continue;
+;       printf("step 2: %d\n", x);
+;     }
+;   }
+;
+; If we PRE %cmp3, CodeGenPrepare won't be able to sink the compare down to its
+; uses, and we are forced to keep both %x and %cmp3 in registers in the loop.
+;
+; It is just as cheap to recompute the icmp against %x as it is to compare a
+; GPR against 0. On x86-64, the br i1 %cmp3 becomes:
+;
+;   testb %r12b, %r12b
+;   jne	LBB0_3
+;
+; The sunk icmp is:
+;
+;   cmpl $2, %ebx
+;   je	LBB0_3
+;
+; This is just as good, and it doesn't require a separate register.
+;
+; CHECK-NOT: phi i1
+
+@.str = private unnamed_addr constant [2 x i8] c"a\00", align 1
+@.str1 = private unnamed_addr constant [2 x i8] c"b\00", align 1
+@.str2 = private unnamed_addr constant [7 x i8] c"step 1\00", align 1
+@.str3 = private unnamed_addr constant [12 x i8] c"step 2: %d\0A\00", align 1
+
+define void @f(i32 %x) noreturn nounwind uwtable ssp {
+entry:
+  %cmp = icmp eq i32 %x, 1
+  br i1 %cmp, label %for.cond.preheader, label %if.then
+
+if.then:                                          ; preds = %entry
+  %cmp1 = icmp eq i32 %x, 2
+  %cond = select i1 %cmp1, i8* getelementptr inbounds ([2 x i8]* @.str, i64 0, i64 0), i8* getelementptr inbounds ([2 x i8]* @.str1, i64 0, i64 0)
+  %call = tail call i32 @puts(i8* %cond) nounwind
+  br label %for.cond.preheader
+
+for.cond.preheader:                               ; preds = %entry, %if.then
+  %cmp3 = icmp eq i32 %x, 2
+  br label %for.cond
+
+for.cond:                                         ; preds = %for.cond.backedge, %for.cond.preheader
+  %call2 = tail call i32 @puts(i8* getelementptr inbounds ([7 x i8]* @.str2, i64 0, i64 0)) nounwind
+  br i1 %cmp3, label %for.cond.backedge, label %if.end5
+
+if.end5:                                          ; preds = %for.cond
+  %call6 = tail call i32 (i8*, ...)* @printf(i8* getelementptr inbounds ([12 x i8]* @.str3, i64 0, i64 0), i32 %x) nounwind
+  br label %for.cond.backedge
+
+for.cond.backedge:                                ; preds = %if.end5, %for.cond
+  br label %for.cond
+}
+
+declare i32 @puts(i8* nocapture) nounwind
+
+declare i32 @printf(i8* nocapture, ...) nounwind