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From: "carrot at google dot com" <gcc-bugzilla@gcc.gnu.org> To: gcc-bugs@gcc.gnu.org Subject: [Bug target/45980] New: Use not in stead of add to generate new constant Date: Tue, 12 Oct 2010 08:42:00 -0000 [thread overview] Message-ID: <bug-45980-4@http.gcc.gnu.org/bugzilla/> (raw) http://gcc.gnu.org/bugzilla/show_bug.cgi?id=45980 Summary: Use not in stead of add to generate new constant Product: gcc Version: 4.6.0 Status: UNCONFIRMED Severity: normal Priority: P3 Component: target AssignedTo: unassigned@gcc.gnu.org ReportedBy: carrot@google.com CC: carrot@google.com Host: i686-linux Target: arm-eabi Build: i686-linux Compile the following code: typedef struct { unsigned long state[5]; unsigned long count[2]; } SHA1_CTX; void SHA1Init(SHA1_CTX* context) { /* SHA1 initialization constants */ context->state[0] = 0x67452301; context->state[1] = 0xEFCDAB89; context->state[2] = 0x98BADCFE; context->state[3] = 0x10325476; context->state[4] = 0xC3D2E1F0; context->count[0] = context->count[1] = 0; } With options -march=armv7-a -mthumb -Os, gcc generates: SHA1Init: ldr r3, .L2 str r3, [r0, #0] add r3, r3, #-2004318072 str r3, [r0, #4] ldr r3, .L2+4 str r3, [r0, #8] sub r3, r3, #-2004318072 str r3, [r0, #12] ldr r3, .L2+8 str r3, [r0, #16] movs r3, #0 str r3, [r0, #24] str r3, [r0, #20] bx lr .L3: .align 2 .L2: .word 1732584193 .word -1732584194 .word -1009589776 This function needs to store 5 large constants to memory. Instead of load the 5 constants from constant pool, gcc found two of them can be computed out by a single add/sub constant instruction. But we can do better, notice that 0x67452301 + 0x98BADCFE = 0xFFFFFFFF 0xEFCDAB89 + 0x10325476 = 0xFFFFFFFF So if we have one such constant, the other one can be computed out by bitwise not. So a shorter result could be: SHA1Init: ldr r3, .L2 str r3, [r0, #0] add r2, r3, #-2004318072 str r2, [r0, #4] movns r3, r3 str r3, [r0, #8] movns r2, r2 str r2, [r0, #12] ldr r3, .L2+4 str r3, [r0, #16] movs r3, #0 str r3, [r0, #24] str r3, [r0, #20] bx lr .L3: .align 2 .L2: .word 1732584193 .word -1009589776
next reply other threads:[~2010-10-12 8:42 UTC|newest] Thread overview: 5+ messages / expand[flat|nested] mbox.gz Atom feed top 2010-10-12 8:42 carrot at google dot com [this message] 2010-10-14 2:14 ` [Bug target/45980] " ramana at gcc dot gnu.org 2010-10-18 6:24 ` carrot at google dot com 2011-03-05 14:40 ` rearnsha at gcc dot gnu.org 2023-05-15 5:10 ` pinskia at gcc dot gnu.org
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