Bug 160329 – ARM64: Fused left shift with a right shift can create NaNs from integers

• Component: JavaScriptCore
• Status: RESOLVED FIXED

[Michael Saboff]

The function 
function signExtendByte(x) {
    return x << 24 >> 24;
}

Generates the wrong instructions when compiled with the FTL JIT.  The shift left / shift right combination becomes a sign extend, which is fine except it sign extends to a 64 bit value and not a 32 bit value.
Here is the code that the FTL generates for “x << 24 >> 24” (with my comments):
         0x10695fdd8:    ldur   x0, [fp, #48]           ; Load x_0
         0x10695fddc:    mov    x1, #0xffff000000000000.; materialize tag
         0x10695fde0:    cmp    x0, x1                  ; Check for int
         0x10695fde4:    b.lo   0x10695feac
         0x10695fde8:    sxtb   x0, w0                  ; sign extend byte to 64 bits (oops should be 32 bits)
         0x10695fdec:    add    x0, x0, x1              ; Add in tag

The sxtb x0, w0 should be a sxtb w0, w0.

[Michael Saboff]

<rdar://problem/27299339>

[Michael Saboff]

Created attachment 284842 [details]
Patch

[Geoffrey Garen]

Comment on attachment 284842 [details]
Patch

View in context: https://bugs.webkit.org/attachment.cgi?id=284842&action=review

r=me

> Source/JavaScriptCore/ChangeLog:10
> +        generate a sign extend byte instructions.  On ARM64, we were sign extending

instruction

> Source/JavaScriptCore/ChangeLog:11
> +        to a 64 bit quantity, when we really wanted to signn extend to a 32 bit quantity.

sign

[Michael Saboff]

Committed r203851: <http://trac.webkit.org/changeset/203851>