[PATCH] SelectionDAG: fold (fp_to_u/sint (u/sint_to_fp val)) when possible

[Fiona Glaser]

This originally came up on IR for an out-of-tree target, but some test C code showed this is actually a general optimization we’re missing. gcc optimizes this one, so I figure we should probably do it too.

The basic idea is that converting from Type A to float to Type B can be optimized out, regardless of float modes (even in safe math!) if:

a) the float can precisely represent the full range of all values in Type A
b) Type B has a strictly equal to or larger range than Type A.

If Type A == Type B we can drop the operation entirely; otherwise we can convert it to a zero_extend or sign_extend depending on the types.

A quick C example for testing that gcc gets but not clang:

int32_t foo(uint8_t x) {
    return (int32_t)(float)x;
}

Fiona

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