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WString: Optimize a bit #7553

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Merged
merged 4 commits into from
Nov 16, 2020
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WString: Optimize a bit #7553

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f2c7565
WString: Optimize a bit
jjsuwa-sys3175 Sep 9, 2020
6e76a3a
* optimize `init()` more
jjsuwa-sys3175 Nov 15, 2020
ebe4ceb
* revise the comment
jjsuwa-sys3175 Nov 15, 2020
df10fff
Merge branch 'master' into WString_cutWaste
d-a-v Nov 16, 2020
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* revise the comment
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@jjsuwa-sys3175
jjsuwa-sys3175 committed Nov 15, 2020
commit ebe4ceb0b102545409eb220cb102a2d630280a20
20 changes: 13 additions & 7 deletions cores/esp8266/WString.h
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Expand Up @@ -324,15 +324,21 @@ class String {

protected:
void init(void) __attribute__((always_inline)) {
sso.buff[0] = 0; // In the Xtensa ISA, in fact, 32-bit store insn ("S32I.N") is one-byte shorter than 8-bit one ("S8I").
sso.buff[1] = 0; // Thanks to store-merging optimization, these 9 lines emit only 3 insns:
sso.buff[2] = 0; // "MOVI.N aX,0", "S32I.N aX,a2,0" and "S32I.N aX,a2,8" (6 bytes in total)
sso.buff[0] = 0;
sso.len = 0;
sso.isHeap = 0;
// Without the 6 statements shown below, GCC simply emits such as: "MOVI.N aX,0", "S8I aX,a2,0" and "S8I aX,a2,11" (8 bytes in total)
sso.buff[1] = 0;
sso.buff[2] = 0;
sso.buff[3] = 0;
sso.buff[8] = 0; // Unfortunately, GCC seems not to re-evaluate the cost of inlining after the store-merging optimizer stage,
sso.buff[9] = 0; // `always_inline` attribute is necessary in order to assure inlining.
sso.buff[8] = 0;
sso.buff[9] = 0;
sso.buff[10] = 0;
sso.len = 0;
sso.isHeap = 0;
// With the above, thanks to store-merging, GCC can use the narrow form of 32-bit store insn ("S32I.N") and emits:
// "MOVI.N aX,0", "S32I.N aX,a2,0" and "S32I.N aX,a2,8" (6 bytes in total)
// (Literature: Xtensa(R) Instruction Set Reference Manual, "S8I - Store 8-bit" [p.504] and "S32I.N - Narrow Store 32-bit" [p.512])
// Unfortunately, GCC seems not to re-evaluate the cost of inlining after the store-merging optimizer stage,
// `always_inline` attribute is necessary in order to keep inlining.
}
void invalidate(void);
unsigned char changeBuffer(unsigned int maxStrLen);
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