The subnormal exponent calculation invokes UB by left shifting the signed exponent to find the first leading bit. The implementation also uses 32 bits operations, which generates suboptimal code in 64 bits architectures. The patch reimplements ilogb using the math_config.h macros and uses the new stdbit function to simplify the subnormal handling. On aarch64 it generates better code: * master: 0000000000000000 <__ieee754_ilogb>: 0: 9e660000 fmov x0, d0 4: d360fc02 lsr x2, x0, #32 8: d360f801 ubfx x1, x0, #32, #31 c: f26c285f tst x2, #0x7ff00000 10: 540001a1 b.ne 44 <__ieee754_ilogb+0x44> // b.any 14: 2a000022 orr w2, w1, w0 18: 34000322 cbz w2, 7c <__ieee754_ilogb+0x7c> 1c: 35000221 cbnz w1, 60 <__ieee754_ilogb+0x60> 20: 2a0003e1 mov w1, w0 24: 7100001f cmp w0, #0x0 28: 12808240 mov w0, #0xfffffbed // #-1043 2c: 540000ad b.le 40 <__ieee754_ilogb+0x40> 30: 531f7821 lsl w1, w1, #1 34: 51000400 sub w0, w0, #0x1 38: 7100003f cmp w1, #0x0 3c: 54ffffac b.gt 30 <__ieee754_ilogb+0x30> 40: d65f03c0 ret 44: 13147c20 asr w0, w1, #20 48: 12b00202 mov w2, #0x7fefffff // #2146435071 4c: 510ffc00 sub w0, w0, #0x3ff 50: 6b02003f cmp w1, w2 54: 12b00001 mov w1, #0x7fffffff // #2147483647 58: 1a819000 csel w0, w0, w1, ls // ls = plast 5c: d65f03c0 ret 60: 53155021 lsl w1, w1, #11 64: 12807fa0 mov w0, #0xfffffc02 // #-1022 68: 531f7821 lsl w1, w1, #1 6c: 51000400 sub w0, w0, #0x1 70: 7100003f cmp w1, #0x0 74: 54ffffac b.gt 68 <__ieee754_ilogb+0x68> 78: d65f03c0 ret 7c: 320107e0 mov w0, #0x80000001 // #-2147483647 80: d65f03c0 ret * patch: 0000000000000000 <__ieee754_ilogb>: 0: 9e660001 fmov x1, d0 4: d374f820 ubfx x0, x1, #52, #11 8: 350000e0 cbnz w0, 24 <__ieee754_ilogb+0x24> c: d374cc21 lsl x1, x1, #12 10: b4000141 cbz x1, 38 <__ieee754_ilogb+0x38> 14: dac01021 clz x1, x1 18: 12807fc0 mov w0, #0xfffffc01 // #-1023 1c: 4b010000 sub w0, w0, w1 20: d65f03c0 ret 24: 711ffc1f cmp w0, #0x7ff 28: 510ffc00 sub w0, w0, #0x3ff 2c: 12b00001 mov w1, #0x7fffffff // #2147483647 30: 1a811000 csel w0, w0, w1, ne // ne = any 34: d65f03c0 ret 38: 320107e0 mov w0, #0x80000001 // #-2147483647 3c: d65f03c0 ret Other architecture with support for stdc_leading_zeros and/or __builtin_clzll should have similar improvements. Checked on aarch64-linux-gnu and x86_64-linux-gnu. Reviewed-by: Wilco Dijkstra <Wilco.Dijkstra@arm.com>

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