Redundant Trivial Basic Blocks Caused by select

Hi all,

I have a simple test case like this

class H



int a;

int b;

int c;


inline int operator < (H& A, H& B)


return (A.a < B.a) ? 1 :

(A.a > B.a) ? 0 :

(A.b < B.b) ? 1 :

(A.b > B.b) ? 0 :

A.c < B.c;


int s(H *h, int j)


if (h[j] < h[j+1])


return j;


The generated assembly in AArch64 is like below. X86 assembly has similar problem.

If(h0.a < h1.a) goto bb0

if (h0.a > h1.a) goto bb1

If(h0.b < h1.b) goto bb2

if (h0.b > h1.b) goto bb3

flag = (h0.c < h1.c)

goto if_end

bb0: Flag = 1; Goto if_end

bb1: Flag = 0; Goto if_end

bb2: Flag = 1; Goto if_end

bb3: Flag = 0; Goto if_end

if_end: j = select flag, j+2, j

The trivial basic blocks bb0 – bb3 can cause huge performance penalty if the comparison (h[j] < h[j+1]) is in a hot loop. The IR code of if_end is like this

%cond = phi i1 [ false, … ], [ true, … ], [ false, … ], [ true, … ], [%flag, …]

%add = add i32 %j, 2

%j.add = select i1 %cond.i, i32 %j, i32 %add

Every bool constant in the phi becomes a trivial basic block in the assembly. select is generated by SpeculativelyExecuteBB() of simplifyCFG before function inlining and prevents jump-threading to further optimize the CFG. If I run the passes in the order of

-inline -instcombine -jump-threading -simplifycfg

then the problem is gone. instcombine can clean the code after inlining so that jump-threading can optimize the CFG.

Please let me know if you have any advice to solve this problem.

Thank you,