Extended Description

this was found by my student Yuyou Fan

regehr@home:~/tmp$ cat reduced.ll
; ModuleID = 'foo.ll'
source_filename = "foo.ll"

define i32 @​src(i64 %x, i32 %nbits) local_unnamed_addr {
%t0 = zext i32 %nbits to i64
%t1 = shl i64 -1, %t0
%t2 = ashr exact i64 %t1, %t0
%t3 = add i32 %nbits, -33
%t4 = and i64 %t2, %x
%t5 = trunc i64 %t4 to i32
%t6 = shl i32 %t5, %t3
ret i32 %t6
}
regehr@home:/tmp$ opt -instcombine reduced.ll -o reduced-opt.ll
regehr@home:/tmp$ ~/alive2/build/alive-tv reduced.ll reduced-opt.ll

define i32 @​src(i64 %x, i32 %nbits) {
%0:
%t0 = zext i32 %nbits to i64
%t1 = shl i64 -1, %t0
%t2 = ashr exact i64 %t1, %t0
%t3 = add i32 %nbits, 4294967263
%t4 = and i64 %t2, %x
%t5 = trunc i64 %t4 to i32
%t6 = shl i32 %t5, %t3
ret i32 %t6
}
=>
define i32 @​src(i64 %x, i32 %nbits) {
%0:
%t3 = add i32 %nbits, 4294967263
%1 = trunc i64 %x to i32
%2 = shl i32 %1, %t3
%t6 = and i32 %2, 2147483647
ret i32 %t6
}
Transformation doesn't verify!

ERROR: Value mismatch

Example:
i64 %x = #x0000000040000000 (1073741824)
i32 %nbits = #x00000022 (34)

Source:
i64 %t0 = #x0000000000000022 (34)
i64 %t1 = #xfffffffc00000000 (18446744056529682432, -17179869184)
i64 %t2 = #xffffffffffffffff (18446744073709551615, -1)
i32 %t3 = #x00000001 (1)
i64 %t4 = #x0000000040000000 (1073741824)
i32 %t5 = #x40000000 (1073741824)
i32 %t6 = #x80000000 (2147483648, -2147483648)

Target:
i32 %t3 = #x00000001 (1)
i32 %1 = #x40000000 (1073741824)
i32 %2 = #x80000000 (2147483648, -2147483648)
i32 %t6 = #x00000000 (0)
Source value: #x80000000 (2147483648, -2147483648)
Target value: #x00000000 (0)

Summary:
0 correct transformations
1 incorrect transformations
0 failed-to-prove transformations
0 Alive2 errors
regehr@home:~/tmp$