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The code that originally made me discover this is:

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if ((a & 0x1) == 0x1) {
    ..
  }

In this case we don't actually have any branch probability information and
should not assume to have any. LLVM transforms this into:

  %and = and i32 %a, 1
  %tobool = icmp eq i32 %and, 0

So, in this case, the result of a bitwise and is compared against 0,
but nevertheless, we should not assume to have probability
information.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@234898 91177308-0d34-0410-b5e6-96231b3b80d8
This commit is contained in:
Daniel Jasper 2015-04-14 15:20:37 +00:00
parent d87c77c0e8
commit 7025d248eb
4 changed files with 39 additions and 3 deletions
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8
lib/Analysis/BranchProbabilityInfo.cpp
View File

@ -379,6 +379,14 @@ bool BranchProbabilityInfo::calcZeroHeuristics(BasicBlock *BB) {
if (!CV)
return false;
// If the LHS is the result of AND'ing a value with a single bit bitmask,
// we don't have information about probabilities.
if (Instruction *LHS = dyn_cast<Instruction>(CI->getOperand(0)))
if (LHS->getOpcode() == Instruction::And)
if (ConstantInt *AndRHS = dyn_cast<ConstantInt>(LHS->getOperand(1)))
if (AndRHS->getUniqueInteger().isPowerOf2())
return false;
bool isProb;
if (CV->isZero()) {
switch (CI->getPredicate()) {

28
test/Analysis/BranchProbabilityInfo/basic.ll
View File

@ -212,3 +212,31 @@ exit:
ret i32 %result
}
define i32 @zero3(i32 %i, i32 %a, i32 %b) {
; CHECK: Printing analysis {{.*}} for function 'zero3'
entry:
; AND'ing with a single bit bitmask essentially leads to a bool comparison,
; meaning we don't have probability information.
%and = and i32 %i, 2
%tobool = icmp eq i32 %and, 0
br i1 %tobool, label %then, label %else
; CHECK: edge entry -> then probability is 16 / 32
; CHECK: edge entry -> else probability is 16 / 32
then:
; AND'ing with other bitmask might be something else, so we still assume the
; usual probabilities.
%and2 = and i32 %i, 5
%tobool2 = icmp eq i32 %and2, 0
br i1 %tobool2, label %else, label %exit
; CHECK: edge then -> else probability is 12 / 32
; CHECK: edge then -> exit probability is 20 / 32
else:
br label %exit
exit:
%result = phi i32 [ %a, %then ], [ %b, %else ]
ret i32 %result
}

2
test/CodeGen/AArch64/arm64-call-tailcalls.ll
View File

@ -53,9 +53,9 @@ bb: ; preds = %entry
define i32 @t8(i32 %x) nounwind ssp {
; CHECK-LABEL: t8:
; CHECK: b _c
; CHECK: b _a
; CHECK: b _b
; CHECK: b _c
%and = and i32 %x, 1
%tobool = icmp eq i32 %and, 0
br i1 %tobool, label %if.end, label %if.then

4
test/CodeGen/Mips/octeon.ll
View File

@ -93,7 +93,7 @@ entry:
; ALL-LABEL: bbit0:
; OCTEON: bbit0 $4, 3, $[[BB0:BB[0-9_]+]]
; MIPS64: andi $[[T0:[0-9]+]], $4, 8
; MIPS64: beqz $[[T0]], $[[BB0:BB[0-9_]+]]
; MIPS64: bnez $[[T0]], $[[BB0:BB[0-9_]+]]
%bit = and i64 %a, 8
%res = icmp eq i64 %bit, 0
br i1 %res, label %endif, label %if
@ -111,7 +111,7 @@ entry:
; MIPS64: daddiu $[[T0:[0-9]+]], $zero, 1
; MIPS64: dsll $[[T1:[0-9]+]], $[[T0]], 35
; MIPS64: and $[[T2:[0-9]+]], $4, $[[T1]]
; MIPS64: beqz $[[T2]], $[[BB0:BB[0-9_]+]]
; MIPS64: bnez $[[T2]], $[[BB0:BB[0-9_]+]]
%bit = and i64 %a, 34359738368
%res = icmp eq i64 %bit, 0
br i1 %res, label %endif, label %if