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9571cb56c5
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
69 lines
2.2 KiB
LLVM
69 lines
2.2 KiB
LLVM
; RUN: opt -gvn -S < %s | FileCheck %s
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; C source:
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;
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; void f(int x) {
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; if (x != 1)
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; puts (x == 2 ? "a" : "b");
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; for (;;) {
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; puts("step 1");
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; if (x == 2)
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; continue;
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; printf("step 2: %d\n", x);
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; }
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; }
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;
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; If we PRE %cmp3, CodeGenPrepare won't be able to sink the compare down to its
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; uses, and we are forced to keep both %x and %cmp3 in registers in the loop.
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;
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; It is just as cheap to recompute the icmp against %x as it is to compare a
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; GPR against 0. On x86-64, the br i1 %cmp3 becomes:
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;
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; testb %r12b, %r12b
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; jne LBB0_3
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;
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; The sunk icmp is:
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;
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; cmpl $2, %ebx
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; je LBB0_3
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;
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; This is just as good, and it doesn't require a separate register.
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;
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; CHECK-NOT: phi i1
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@.str = private unnamed_addr constant [2 x i8] c"a\00", align 1
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@.str1 = private unnamed_addr constant [2 x i8] c"b\00", align 1
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@.str2 = private unnamed_addr constant [7 x i8] c"step 1\00", align 1
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@.str3 = private unnamed_addr constant [12 x i8] c"step 2: %d\0A\00", align 1
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define void @f(i32 %x) noreturn nounwind uwtable ssp {
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entry:
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%cmp = icmp eq i32 %x, 1
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br i1 %cmp, label %for.cond.preheader, label %if.then
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if.then: ; preds = %entry
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%cmp1 = icmp eq i32 %x, 2
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%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)
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%call = tail call i32 @puts(i8* %cond) nounwind
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br label %for.cond.preheader
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for.cond.preheader: ; preds = %entry, %if.then
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%cmp3 = icmp eq i32 %x, 2
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br label %for.cond
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for.cond: ; preds = %for.cond.backedge, %for.cond.preheader
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%call2 = tail call i32 @puts(i8* getelementptr inbounds ([7 x i8]* @.str2, i64 0, i64 0)) nounwind
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br i1 %cmp3, label %for.cond.backedge, label %if.end5
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if.end5: ; preds = %for.cond
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%call6 = tail call i32 (i8*, ...)* @printf(i8* getelementptr inbounds ([12 x i8]* @.str3, i64 0, i64 0), i32 %x) nounwind
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br label %for.cond.backedge
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for.cond.backedge: ; preds = %if.end5, %for.cond
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br label %for.cond
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}
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declare i32 @puts(i8* nocapture) nounwind
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declare i32 @printf(i8* nocapture, ...) nounwind
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