Use fixed-point representation for BranchProbability.
BranchProbability now is represented by its numerator and denominator in uint32_t type. This patch changes this representation into a fixed point that is represented by the numerator in uint32_t type and a constant denominator 1<<31. This is quite similar to the representation of BlockMass in BlockFrequencyInfoImpl.h. There are several pros and cons of this change:
Pros:
1. It uses only a half space of the current one.
2. Some operations are much faster like plus, subtraction, comparison, and scaling by an integer.
Cons:
1. Constructing a probability using arbitrary numerator and denominator needs additional calculations.
2. It is a little less precise than before as we use a fixed denominator. For example, 1 - 1/3 may not be exactly identical to 1 / 3 (this will lead to many BranchProbability unit test failures). This should not matter when we only use it for branch probability. If we use it like a rational value for some precise calculations we may need another construct like ValueRatio.
One important reason for this change is that we propose to store branch probabilities instead of edge weights in MachineBasicBlock. We also want clients to use probability instead of weight when adding successors to a MBB. The current BranchProbability has more space which may be a concern.
Differential revision: http://reviews.llvm.org/D12603
llvm-svn: 248633
diff --git a/llvm/test/Transforms/SampleProfile/propagate.ll b/llvm/test/Transforms/SampleProfile/propagate.ll
index 7d5ce73..ca58d17 100644
--- a/llvm/test/Transforms/SampleProfile/propagate.ll
+++ b/llvm/test/Transforms/SampleProfile/propagate.ll
@@ -73,8 +73,8 @@
%5 = load i64, i64* %N.addr, align 8, !dbg !15
%cmp1 = icmp slt i64 %4, %5, !dbg !15
br i1 %cmp1, label %for.body, label %for.end18, !dbg !15
-; CHECK: edge for.cond -> for.body probability is 10 / 10 = 100% [HOT edge]
-; CHECK: edge for.cond -> for.end18 probability is 0 / 10 = 0%
+; CHECK: edge for.cond -> for.body probability is 0x80000000 / 0x80000000 = 100.00% [HOT edge]
+; CHECK: edge for.cond -> for.end18 probability is 0x00000000 / 0x80000000 = 0.00%
for.body: ; preds = %for.cond
%6 = load i64, i64* %i, align 8, !dbg !18
@@ -82,8 +82,8 @@
%div = sdiv i64 %7, 3, !dbg !18
%cmp2 = icmp sgt i64 %6, %div, !dbg !18
br i1 %cmp2, label %if.then3, label %if.end, !dbg !18
-; CHECK: edge for.body -> if.then3 probability is 1 / 5 = 20%
-; CHECK: edge for.body -> if.end probability is 4 / 5 = 80%
+; CHECK: edge for.body -> if.then3 probability is 0x1999999a / 0x80000000 = 20.00%
+; CHECK: edge for.body -> if.end probability is 0x66666666 / 0x80000000 = 80.00%
if.then3: ; preds = %for.body
%8 = load i32, i32* %x.addr, align 4, !dbg !21
@@ -97,8 +97,8 @@
%div4 = sdiv i64 %10, 4, !dbg !22
%cmp5 = icmp sgt i64 %9, %div4, !dbg !22
br i1 %cmp5, label %if.then6, label %if.else7, !dbg !22
-; CHECK: edge if.end -> if.then6 probability is 3 / 6342 = 0.0473037%
-; CHECK: edge if.end -> if.else7 probability is 6339 / 6342 = 99.9527% [HOT edge]
+; CHECK: edge if.end -> if.then6 probability is 0x000f801f / 0x80000000 = 0.05%
+; CHECK: edge if.end -> if.else7 probability is 0x7ff07fe1 / 0x80000000 = 99.95% [HOT edge]
if.then6: ; preds = %if.end
%11 = load i32, i32* %y.addr, align 4, !dbg !24
@@ -119,8 +119,8 @@
%14 = load i64, i64* %i, align 8, !dbg !28
%cmp10 = icmp slt i64 %conv9, %14, !dbg !28
br i1 %cmp10, label %for.body11, label %for.end, !dbg !28
-; CHECK: edge for.cond8 -> for.body11 probability is 16191 / 16191 = 100% [HOT edge]
-; CHECK: edge for.cond8 -> for.end probability is 0 / 16191 = 0%
+; CHECK: edge for.cond8 -> for.body11 probability is 0x80000000 / 0x80000000 = 100.00% [HOT edge]
+; CHECK: edge for.cond8 -> for.end probability is 0x00000000 / 0x80000000 = 0.00%
for.body11: ; preds = %for.cond8
%15 = load i32, i32* %j, align 4, !dbg !31