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[libc] Add support for MVE to Arm startup code #167338

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Merged
merged 3 commits into from
Nov 12, 2025
Merged

[libc] Add support for MVE to Arm startup code #167338

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[libc] Add support for MVE to Arm startup code
vhscampos Nov 10, 2025
77b94bd
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vhscampos Nov 11, 2025
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vhscampos Nov 12, 2025
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27 changes: 18 additions & 9 deletions libc/startup/baremetal/arm/start.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -131,30 +131,39 @@ namespace LIBC_NAMESPACE_DECL {
__arm_wsr("CPSR_c", 0x13); // SVC
#endif

#ifdef __ARM_FP
// Enable FPU
#if __ARM_ARCH_PROFILE == 'M'
#if __ARM_ARCH_PROFILE == 'M' && \
(defined(__ARM_FP) || defined(__ARM_FEATURE_MVE))
// Enable FPU and MVE. They can't be enabled independently: the two are
// governed by the same bits in CPACR.
// Based on
// https://developer.arm.com/documentation/dui0646/c/Cortex-M7-Peripherals/Floating-Point-Unit/Enabling-the-FPU
// Set CPACR cp10 and cp11
auto cpacr = (volatile uint32_t *const)0xE000ED88;
// Set CPACR cp10 and cp11.
auto cpacr = reinterpret_cast<volatile uint32_t *const>(0xE000ED88);
*cpacr |= (0xF << 20);
__dsb(0xF);
__isb(0xF);
#elif __ARM_ARCH_PROFILE == 'A' || __ARM_ARCH_PROFILE == 'R'
#if defined(__ARM_FEATURE_MVE)
// Initialize low-overhead-loop tail predication to its neutral state
uint32_t fpscr;
__asm__ __volatile__("vmrs %0, FPSCR" : "=r"(fpscr) : :);
fpscr |= (0x4 << 16);
__asm__ __volatile__("vmsr FPSCR, %0" : : "r"(fpscr) :);
#endif
#elif (__ARM_ARCH_PROFILE == 'A' || __ARM_ARCH_PROFILE == 'R') && \
defined(__ARM_FP)
// Enable FPU.
// Based on
// https://developer.arm.com/documentation/dui0472/m/Compiler-Coding-Practices/Enabling-NEON-and-FPU-for-bare-metal
// Set CPACR cp10 and cp11
// Set CPACR cp10 and cp11.
uint32_t cpacr = __arm_rsr("p15:0:c1:c0:2");
cpacr |= (0xF << 20);
__arm_wsr("p15:0:c1:c0:2", cpacr);
__isb(0xF);
// Set FPEXC.EN
uint32_t fpexc;
__asm__ __volatile__("vmrs %0, FPEXC" : "=r"(fpexc) : :);
fpexc |= (1 << 30);
fpexc |= (0x1 << 30);
__asm__ __volatile__("vmsr FPEXC, %0" : : "r"(fpexc) :);
#endif
#endif

// Perform the equivalent of scatterloading
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