miscompile due to instcombine

[regehr]

https://alive2.llvm.org/ce/z/w-iYiR

so if we compile this function:

define i32 @t1_ult_slt_0(i32 %x, i32 %replacement_low, i32 %replacement_high) {
  %t0 = icmp slt i32 %x, 0
  %t1 = select i1 %t0, i32 %replacement_low, i32 %replacement_high
  %t2 = icmp ult i32 %x, 65536
  %1 = xor i1 %t2, true
  %r = select i1 %1, i32 %x, i32 %t1
  ret i32 %r
}

alongside this test driver:

#include <stdio.h>

unsigned t1_ult_slt_0(unsigned a, unsigned b, unsigned c);

int main(void) {
  printf("%u\n", t1_ult_slt_0(2, 0, 1));
}

the output is 1

optimizing the function using instcombine we get this:

regehr@john-home:~/Downloads$ opt -instcombine foo.ll -S -o -
; ModuleID = 'foo.ll'
source_filename = "foo.ll"

define i32 @t1_ult_slt_0(i32 %x, i32 %replacement_low, i32 %replacement_high) {
  %1 = icmp slt i32 %x, 0
  %2 = icmp sgt i32 %x, 65535
  %3 = select i1 %1, i32 %replacement_low, i32 %x
  %4 = select i1 %2, i32 %replacement_high, i32 %3
  ret i32 %4
}
regehr@john-home:~/Downloads$ 

compiling the optimized function, using the same test driver, results in 2 being printed

found by @Hatsunespica's test generater + Alive2
cc @nunoplopes

[nunoplopes]

cc @rotateright @nikic @LebedevRI

[rickyz]

Sent out https://reviews.llvm.org/D119690 to fix this.

[fhahn]

Thanks!